There are actually two variants of ALC268 Acer implementation, one
with an analog built-in mic (pin 0x19) and another with a digital
mic (pin 0x12).  Created a new model, acer-dmic, for the latter case
now.

So far, all known models are assigned to be analog-mic, according to
the BIOS setup.  If this doesn't match with the actual case, one needs
to try model=acer-dmic, and fix the entry to point ALC268_ACER_DMIC
if it works.

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