V4L/DVB (6833): Don't limit cx88 audio routing to blackbird boards

All cx2388 boards need the line-in audio to be routed from an external ADC
(refered to as "ADC mode" in the spec sheet), since the chip is uncapable
of dealing with baseband audio directly.
So... this patch enables normal mode when using the tuner (TV or Radio), and
enables ADC mode with any other source. It'll probably only work with boards
that have supported ADCs (such as the Wolfson wm9775)

Signed-off-by: Ricardo Cerqueira <v4l@cerqueira.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>