[media] si2157: cancel_delayed_work_sync before device removal / kfree

si2157_remove  was  calling  kfree(dev)  with  possibly  still  active
schedule_delayed_work(dev->stat_work).  This  caused kernel  panics in
call_timer_fn e.g. after rmmod cx23885.

Signed-off-by: Ernst Martin Witte <emw-linux-kernel@nocabal.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>