V4L/DVB (10645): vivi: introduce v4l2_device and do several cleanups

- add v4l2_device
- remove BKL
- make the debug parameter settable on the fly
- set bus_info in querycap
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>