[media] cx231xx: Ensure VBI fields are sent in the correct order

The current code was sending one videobuf per field (despite having specified
V4L2_FIELD_SEQ_TB during setup).  As a result, application which used the
read() interface would work, except they would sometimes have the fields
reversed (depending on the luck of which field the device was on when the
application started VBI capture).  The net effect was that CC decoding would
only work about 50% of the time.

Restructure the VBI code a bit so that works like all the other drivers, such
that both fields are delivered in a single videobuf buffer, which ensures that
they are always received in a predictable order.

Signed-off-by: Devin Heitmueller <dheitmueller@hauppauge.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>