Cleanup v4l2-dev.[ch], add/improve comments and add a new helper
function: video_drvdata() that can get the private driver data from
a file struct.
Signed-off-by: NHans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>