[media] au0828: Cancel stream-restart operation if frontend is disconnected

If the tuner is already disconnected, It is meaningless to go on doing the
stream-restart operation, It is better to cancel this operation.

Signed-off-by: Changbing Xiong <cb.xiong@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>