V4L/DVB (11703): cx18: Have audio decoder drive SIF gain control, and rework AFE config

Ensure the variable gain amplifier gain for SIF is driven by the audio deocder
and not the video decoder.  This forced rework of the analog front end (AFE)
configuration to not rely on autoconfiguration, but instead set up the AFE mux,
AFE parameters, and ADC1 & ADC2 configurations explicitly.

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