V4L/DVB: v4l: move vbi-specific video ops to a new vbi ops struct

Only a relatively small number of video receivers and transmitters actually
support VBI. So start moving the vbi specific ops to an ops struct of their
own.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>