Add a bitmask to hda_gen_spec indicating NIDs to exclude from the
possible volume controls.  That is, when the bit is set, the NID
corresponding to the bit won't be picked as an output volume control
any longer.

Basically this is just a band-aid for working around the issue found
with CS4208 codec, where only the headphone pin has a volume AMP with
different dB steps.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=60811
Cc: <stable@vger.kernel.org> [v3.12+]
Signed-off-by: NTakashi Iwai <tiwai@suse.de>