ALSA: hda - Treat stereo-to-mono mix properly

The commit [ef403edb: ALSA: hda - Don't access stereo amps for
mono channel widgets] fixed the handling of mono widgets in general,
but it still misses an exceptional case: namely, a mono mixer widget
taking a single stereo input.  In this case, it has stereo volumes
although it's a mono widget, and thus we have to take care of both
left and right input channels, as stated in HD-audio spec ("7.1.3
Widget Interconnection Rules").

This patch covers this missing piece by adding proper checks of stereo
amps in both the generic parser and the proc output codes.

Reported-by: Raymond Yau <superquad.vortex2@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>