V4L/DVB (13553): v4l: Use the video_is_registered function in device drivers

Fix all device drivers to use the video_is_registered function instead
of checking video_device::minor.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>