V4L/DVB (13556): 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: removed tm6000 changes as tm6000 is not ready yet for submission even on staging]

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