ALSA: hda - Mute all aamix inputs as default

Not all channels have been initialized, so far, especially when aamix
NID itself doesn't have amps but its leaves have.  This patch fixes
these holes.  Otherwise you might get unexpected loopback inputs,
e.g. from surround channels.

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