ALSA: hda - Allow aamix as a capture source

Since some codecs can choose the aamix as a capture source, we should
support it as well.  When spec->add_stereo_mix_input flag is set, the
parser checks the availability of aamix as the input source, and adds
the paths automatically when possible.

Signed-off-by: Takashi Iwai <tiwai@suse.de>