V4L/DVB (13552): v4l: Replace video_is_unregistered with video_is_registered

Replace the video_is_unregistered function by a video_is_registered
function. The V4L2_FL_UNREGISTERED flag is replaced by a
V4L2_FL_REGISTERED flag.

This change makes the video_is_registered function return coherent
results when called on an initialize but not yet registered video_device
instance. The function can now be used instead of checking
video_device::minor.

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