[media] bw-qcam: convert to videobuf2

I know, nobody really cares about this black-and-white webcam anymore, but
it was fun to do.
Tested with an actual webcam.

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