V4L/DVB (8784): v4l2-dev: make the video_device's release callback mandatory

Now that all drivers set the release callback in the video_device
struct we can put in a BUG_ON in video_register_device to ensure that
the callback is always there.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>