[ALSA] hda-codec - add snd_hda_codec_stereo() function

Added snd_hda_codec_amp_stereo() function that changes both of stereo
channels with the same mask and value bits.  It simplifies most of
amp-handling codes.

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