V4L/DVB (13556a): v4l: Remove unneeded video_device::minor assignments

Now that the video_device registration is tested using
video_is_registered(), drivers don't need to initialize the
video_device::minor field to -1 anymore.

Remove those unneeded assignments.

[mchehab.redhat.com: apply just the tm6000 changes on a separate patch]

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