[media] cx25840: fix VBI support for cx23888

The cx23888 has the VBI registers at different addresses compared to
the other variants. In most cases it is a fixed offset, but not always.

Update the code so the right registers are written for the cx23888.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>