[ALSA] hda-codec - make volume knob, the master volume for sigmatel codecs

VolumeKnob is present on most sigmatel codecs, it allows to decrease
volume of all DACs at once, it is a kind of post-procesing volume.
Note that all output amps of sigmatel only decrease volume, and all
input amps only increase volume.

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>