[ALSA] hda-codec - Add independent headphone volume control

This patch addes the support of the independent 'Headphone' volume
control to the generic codec parser.  Some codecs (e.g. Conexant)
have separate connections to the headphone and the independent amp
adjustment is needed.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>