ALSA: HDA: Add external mic quirk for Asus Zenbook UX31E

According to the reporter, external mic starts to work if the
laptop-dmic model is used. According to BIOS pin config, all
pins are consistent with the alc269vb_laptop_dmic fixup, except
for the external mic, which is not present.

Cc: stable@kernel.org
BugLink: https://bugs.launchpad.net/bugs/950490


Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>