au0828: fix VBI handling when in V4L2 streaming mode

It turns up V4L2 streaming mode (a.k.a mmap) was broken for VBI streaming.
This was causing libzvbi to fall back to V4L1 capture mode, and is a blatent
violation of the V4L2 specification.

Make the implementation work properly in this mode.
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>