V4L/DVB (4759): Cx88: use external adc for rca audio inputs on the ASUS PVR-416

For the ASUS PVR-416, the external adc must be used for
the rca audio inputs, but television / radio inputs use
the internal adc.
Thanks to Alex Deucher for lending his card to me.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>