[media] vivi: remove pointless video_nr++

Remove the pointless video_nr++. It doesn't do anything useful and it has
the unexpected side-effect of changing the video_nr module option, so
cat /sys/module/vivi/parameters/video_nr gives a different value back
then what was specified with modprobe.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>