net: qcom/emac: Fix use after free bug in emac_remove due to race condition

In emac_probe, &adpt->work_thread is bound with
emac_work_thread. Then it will be started by timeout
handler emac_tx_timeout or a IRQ handler emac_isr.

If we remove the driver which will call emac_remove
  to make cleanup, there may be a unfinished work.

The possible sequence is as follows:

Fix it by finishing the work before cleanup in the emac_remove
and disable timeout response.

CPU0                  CPU1

                    |emac_work_thread
emac_remove         |
free_netdev         |
kfree(netdev);      |
                    |emac_reinit_locked
                    |emac_mac_down
                    |//use netdev
Fixes: b9b17debc69d ("net: emac: emac gigabit ethernet controller driver")
Signed-off-by: Zheng Wang <zyytlz.wz@163.com>

Signed-off-by: David S. Miller <davem@davemloft.net>
