media: cx18: missing CAP_AUDIO for vbi stream

The v4l2-compliance test complained about this. You can change the
input connector for the vbi stream, but that means that you can have to
be able to do the same for the audio input.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>