ALSA: hda - Fix missing path between aamix and outputs in AD codecs

AD1988 family and AD1882 codecs have another mixer widget (0x21)
between the analog-loopback mixer widget (0x20) and the actual
outputs.  Due to this hole, the analog-loopbacks aren't sent properly
to the output pins.

As a band-aid fix, introduce another fields holding the aamix merge
path, and activate it.

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