ALSA: hda - Fix the detection of "Loopback Mixing" control for VIA codecs

Currently the driver checks only the out_mix_path[] for the primary
output route for judging whether to create the loopback-mixing control
or not.  But, there are cases where aamix-routing is available only on
headphone or speaker paths but not on the primary output path.  So, the
driver ignores such cases inappropriately.

This patch fixes the check of the loopback-mixing control by testing
all mix-routing paths.

Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>