[media] cx23885: hook the audio selection functions into the main driver

Ensure audio is established at driver start.
Ensure the correct defaults are established for the audio path if
the cards struct has nothing defined.
Allow the caller to select one of the two baseband input paths.

Signed-off-by: Steven Toth <stoth@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>