[media] au8522: remove note about VBI not being implemented

I got this working a couple of years ago.  Remove it from the
list of known issues.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>