V4L/DVB (8919): cx18: Fix tuner audio input for Compro H900 cards

Earlier fixes to get the tuner audio working correctly broke the audio
on the Compro VideoMate H900 cards. This is now fixed.

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