V4L/DVB: unlock on error path

If we return directly here then we miss out on some mutex_unlock()s

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>