[media] dt3155v4l: fix incorrect mutex locking

A mutex_unlock was missing in the 'success' path of the open() call,
and also at one error path in the same function.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>