Added a new option "snoop" for the traffic control of the HD-audio
controller chip.  When set to 0, the non-snooping mode is used with
the traffic control bit is set in each stream control register.
This may allow better operations in the low power mode, but the actual
implementation is depending pretty much on the chipset.

As already implemented, more or less each chipset has own snoop-control
register bit.  Now this setup refers to the snoop option, too.

Also, a new VIA chipset may require the non-snooping mode when set so
in BIOS.  In such a case, the option value is overridden.

As default, it's still set to snoop=1 for keeping the same behavior as
before.  In near future, it'll be set to 0 as default after checking
it works in every system well.

Signed-off-by: Takashi Iwai <tiwai@suse.de>