ALSA: hda - Fix digital-mic mono recording on ASUS Eee PC

The digital-mic unit on ASUS Eee PC gives PDM signals instead of the
normal stereo PCM, thus you can't record a mono stream from the stereo
stream as is; the summed stereo signal results in almost zero level, and
you'll hear only soft noise.

As a workaround, use ALC269-specific COEF to manipulate the dmic route
for mono, like used for ALC271x.  This is implemented as a fix-up, thus
it works only with model=auto or without REALTEK_QUIRKS Kconfig.

Reported-and-tested-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>