V4L/DVB (10276): cx18, cx2341x, ivtv: Add AC-3 audio encoding control to cx18

Initial addition of controls to set AC-3 audio encoding for the CX23418 - it
does not work yet due to firmware or cx18 driver issues.  This change affects
the common cx2341x and ivtv modules due to shared structures and
common functions.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>