V4L/DVB (11863): cx18: Initial attempt to get sliced VBI working for 625 line systems

Initial changes to get sliced VBI for 625 line system working.  This is patch
is untested.

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