media: em28xx: Fix use-after-free when disconnecting

Fix bug by moving the i2c_unregister_device calls after deregistration
of dvb frontend.

The new style i2c drivers already destroys the frontend object at
i2c_unregister_device time.
When the dvb frontend is unregistered afterwards it leads to this oops:

  [ 6058.866459] BUG: unable to handle kernel NULL pointer dereference at 00000000000001f8
  [ 6058.866578] IP: dvb_frontend_stop+0x30/0xd0 [dvb_core]
  [ 6058.866644] PGD 0
  [ 6058.866646] P4D 0

  [ 6058.866726] Oops: 0000 [#1] SMP
  [ 6058.866768] Modules linked in: rc_pinnacle_pctv_hd(O) em28xx_rc(O) si2157(O) si2168(O) em28xx_dvb(O) em28xx(O) si2165(O) a8293(O) tda10071(O) tea5767(O) tuner(O) cx23885(O) tda18271(O) videobuf2_dvb(O) videobuf2_dma_sg(O) m88ds3103(O) tveeprom(O) cx2341x(O) v4l2_common(O) dvb_core(O) rc_core(O) videobuf2_memops(O) videobuf2_v4l2(O) videobuf2_core(O) videodev(O) media(O) bluetooth ecdh_generic ums_realtek uas rtl8192cu rtl_usb rtl8192c_common rtlwifi usb_storage snd_hda_codec_realtek snd_hda_codec_hdmi snd_hda_codec_generic i2c_mux snd_hda_intel snd_hda_codec snd_hwdep x86_pkg_temp_thermal snd_hda_core kvm_intel kvm irqbypass [last unloaded: videobuf2_memops]
  [ 6058.867497] CPU: 2 PID: 7349 Comm: kworker/2:0 Tainted: G        W  O    4.13.9-gentoo #1
  [ 6058.867595] Hardware name: MEDION E2050 2391/H81H3-EM2, BIOS H81EM2W08.308 08/25/2014
  [ 6058.867692] Workqueue: usb_hub_wq hub_event
  [ 6058.867746] task: ffff88011a15e040 task.stack: ffffc90003074000
  [ 6058.867825] RIP: 0010:dvb_frontend_stop+0x30/0xd0 [dvb_core]
  [ 6058.867896] RSP: 0018:ffffc90003077b58 EFLAGS: 00010293
  [ 6058.867964] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 000000010040001f
  [ 6058.868056] RDX: ffff88011a15e040 RSI: ffffea000464e400 RDI: ffff88001cbe3028
  [ 6058.868150] RBP: ffffc90003077b68 R08: ffff880119390380 R09: 000000010040001f
  [ 6058.868241] R10: ffffc90003077b18 R11: 000000000001e200 R12: ffff88001cbe3028
  [ 6058.868330] R13: ffff88001cbe68d0 R14: ffff8800cf734000 R15: ffff8800cf734098
  [ 6058.868419] FS:  0000000000000000(0000) GS:ffff88011fb00000(0000) knlGS:0000000000000000
  [ 6058.868511] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [ 6058.868578] CR2: 00000000000001f8 CR3: 00000001113c5000 CR4: 00000000001406e0
  [ 6058.868662] Call Trace:
  [ 6058.868705]  dvb_unregister_frontend+0x2a/0x80 [dvb_core]
  [ 6058.868774]  em28xx_dvb_fini+0x132/0x220 [em28xx_dvb]
  [ 6058.868840]  em28xx_close_extension+0x34/0x90 [em28xx]
  [ 6058.868902]  em28xx_usb_disconnect+0x4e/0x70 [em28xx]
  [ 6058.868968]  usb_unbind_interface+0x6d/0x260
  [ 6058.869025]  device_release_driver_internal+0x150/0x210
  [ 6058.869094]  device_release_driver+0xd/0x10
  [ 6058.869150]  bus_remove_device+0xe4/0x160
  [ 6058.869204]  device_del+0x1ce/0x2f0
  [ 6058.869253]  usb_disable_device+0x99/0x270
  [ 6058.869306]  usb_disconnect+0x8d/0x260
  [ 6058.869359]  hub_event+0x93d/0x1520
  [ 6058.869408]  ? dequeue_task_fair+0xae5/0xd20
  [ 6058.869467]  process_one_work+0x1d9/0x3e0
  [ 6058.869522]  worker_thread+0x43/0x3e0
  [ 6058.869576]  kthread+0x104/0x140
  [ 6058.869602]  ? trace_event_raw_event_workqueue_work+0x80/0x80
  [ 6058.869640]  ? kthread_create_on_node+0x40/0x40
  [ 6058.869673]  ret_from_fork+0x22/0x30
  [ 6058.869698] Code: 54 49 89 fc 53 48 8b 9f 18 03 00 00 0f 1f 44 00 00 41 83 bc 24 04 05 00 00 02 74 0c 41 c7 84 24 04 05 00 00 01 00 00 00 0f ae f0 <48> 8b bb f8 01 00 00 48 85 ff 74 5c e8 df 40 f0 e0 48 8b 93 f8
  [ 6058.869850] RIP: dvb_frontend_stop+0x30/0xd0 [dvb_core] RSP: ffffc90003077b58
  [ 6058.869894] CR2: 00000000000001f8
  [ 6058.875880] ---[ end trace 717eecf7193b3fc6 ]---

Signed-off-by: Matthias Schwarzott <zzam@gentoo.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
