[media] cx23885: fix VBI support

Tested VBI support and discovered that the wrong offset was used.
After this change it is now working. Verified with CC/XDS for NTSC
and WSS/Teletext on PAL.

It also reported the wrong start lines for the second field. That's
now fixed as well.

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