V4L/DVB (10277): cx18, cx2341x: Fix bugs in cx18 AC3 control and comply with V4L2 spec

Fix bugs in the cx18 AC3 control implementation that would have affected
ivtv and other drivers via the cx2341x module.  Bring AC3 controls
behavior into comliance with V4L2 specification.  Thanks to Hans Verkuil
for reviewing the previous patch and pointing out the problems.

Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>