V4L/DVB (10441): cx18: Fix VBI ioctl() handling and Raw/Sliced VBI state management

More sliced VBI fixes to bring the cx18 driver closer to full V4L2 spec
compliance for VBI and to get sliced VBI working better.

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