[media] xc4000: added audio_std module parameter

The 'audio_std' module parameter makes it possible to fine tune
some audio related aspects of the driver, like setting the exact
audio standard (NICAM, A2, etc.) to be used for some video standards.

Signed-off-by: Istvan Varga <istvan_v@mailbox.hu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>