usb: fix usb-net segfault

The dev->config pointer isn't set until guest
system initializes usb devices (via
usb_desc_set_config). However qemu networking can
go through some motions prior to that, e.g.:

 #0  is_rndis (s=0x555557261970) at hw/usb/dev-network.c:653
 #1  0x000055555585f723 in usbnet_can_receive (nc=0x55555641e820) at hw/usb/dev-network.c:1315
 #2  0x000055555587635e in qemu_can_send_packet (sender=0x5555572660a0) at net/net.c:470
 #3  0x0000555555878e34 in net_hub_port_can_receive (nc=0x5555562d7800) at net/hub.c:101
 #4  0x000055555587635e in qemu_can_send_packet (sender=0x5555562d7980) at net/net.c:470
 #5  0x000055555587dbca in tap_can_send (opaque=0x5555562d7980) at net/tap.c:172

The command to reproduce most reliably was:

 qemu-system-i386 -usb -device usb-net,vlan=0 -net tap,vlan=0

This wasn't strictly a problem with tap. Other
networking endpoints (vde, user) could trigger
this problem as well.

Fixes: https://bugs.launchpad.net/qemu/+bug/1050823
Cc: qemu-stable@nongnu.org
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
