V4L/DVB (10439): cx18: Clean-up and enable sliced VBI handling

Removed legacy ivtv state variables, added comments, and cleaned
up sliced VBI related code.  Enabled sliced VBI.

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