V4L/DVB (7928): cx18: fix audio registers 808 and 80c

The handling of the audio registers 808 and 80c were based on old datasheets.
Updated to the latest information.

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