net/usb: cancel pending work when unbinding smsc75xx

Cancel pending work before freeing smsc75xx private data structure
during binding. This fixes the following crash in the driver:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000050
IP: mutex_lock+0x2b/0x3f
<snipped>
Workqueue: events smsc75xx_deferred_multicast_write [smsc75xx]
task: ffff8caa83e85700 task.stack: ffff948b80518000
RIP: 0010:mutex_lock+0x2b/0x3f
<snipped>
Call Trace:
 smsc75xx_deferred_multicast_write+0x40/0x1af [smsc75xx]
 process_one_work+0x18d/0x2fc
 worker_thread+0x1a2/0x269
 ? pr_cont_work+0x58/0x58
 kthread+0xfa/0x10a
 ? pr_cont_work+0x58/0x58
 ? rcu_read_unlock_sched_notrace+0x48/0x48
 ret_from_fork+0x22/0x40

Signed-off-by: Yu Zhao <yuzhao@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>