V4L/DVB (6605): Add a modprobe option to manually select audio standard

While there's no public API to define audio standard, adds a hack option for
select them. This is needed only for NICAM and A2 firmwares, since AM, BTSC and
EAIJ are already properly handled, on firmware version 2.7.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>