ALC283-based Chromebook suffers from occasional white noise, and it
turned out that this comes from AA-loopback.  Disable this output path
by just clearing mixer_nid, then the generic parser will skip the
creation of AA-loopback path.
Reported-and-tested-by: Kailang Yang <kailang@realtek.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>