ALSA: hda - Turn off loopback mixing as default

So far, we enabled the loopback mixing control as default, as this
behavior made somewhat compatible with the earlier HD-audio drivers
for Realtek & co.  However, it's getting annoying as we've got more
and more bug reports about the noise coming from the loopback route.
Since the loopback mixing is used fairly rarely and often harmful
(e.g. using PA), let's get rid of the default turn-on lines.

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