V4L/DVB (9269): cx88: add I2S-ADC tvaudio method

This adds I2S-ADC tvaudio mode as a formal method of audio
delivery.

This fixes one bug and adds fm audio via I2S-ADC on cards
that support it.

The bug occured before when I2S-ADC mode was initiated on
composite/s-video open but was then reset within 500ms
by the audio thread which used any previous audio tuning
details.

Signed-off-by: Darron Broad <darron@kewl.org>
Signed-off-by: Steven Toth <stoth@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>