[media] rtl2830: cancel_delayed_work_sync before device removal / kfree

rtl2830_remove  was  calling  kfree(dev) with  possibly  still  active
schedule_delayed_work(&dev->stat_work).   A  similar   bug  in  si2157
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>