On some Realtek codecs like ALC882 or ALC883, the capture source is
no mux but sum widget.  We have to initialize all channels properly
for this type, otherwise noises may come in from the unused route.

The patch assures to mute unused routes, and unmute the currently
selected route.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Daniel J Blueman <daniel.blueman@gmail.com>