[media] dvb_usb_v2: use lock to sync feed and frontend control

There was synchronization problem when streaming was stopped.
Frontend was ran down before stream which causes problems.
Use mutex to synchronization. Now it looks like that:
LOCK
start frontend
UNLOCK
LOCK
start streaming
[...]
stop streaming
UNLOCK
LOCK
stop frontend
UNLOCK

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>