V4L/DVB (9806): cx18: Enable raw VBI capture

A combined authorship patch from Hans Verkuil and Andy Walls.  Raw
VBI can now be captured but requires a video capture to be in progress as well.

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