media: bttv: add back vbi hack

The old (now removed) videobuf framework had an optional vbi hack where
the sequence number of the frame counter was copied in the last 4 bytes
of the buffer. This hack was active only for the read() interface
(so not for streaming I/O), and it was enabled by bttv. This allowed
applications that used read() for the VBI data to match it with the
corresponding video frame.

When bttv was converted to vb2 this hack was forgotten, but some old
applications rely on this.

So add this back, but this time in the bttv driver rather than in the
vb2 framework.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: b7ec3212 ("media: bttv: convert to vb2")
Tested-by: Dr. David Alan Gilbert <dave@treblig.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>