The Acer Aspire AO756 has an analog built-in mic on nid 0x1b and an
external mic on nid 0x19.  The BIOS doesn't set this up.

The mic detect on this Acer Aspire netbook and Acer C7 ChromeBook is
only valid when the headphone is plugged.  The detect circuit relies on
the tip detect switch being closed on the jack.  Tell hda_jack to ignore
the mic sense unless the headphones are plugged.

Signed-off-by: Dylan Reid <dgreid@chromium.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>