ALSA: hda - Force AD1988_6STACK_DIG for Asus M3N-HT Deluxe

The microphone input on the back panel (pink connector)
stopped operating correctly after an upgrade from
2.6.35 to 2.6.38; the actual problem manifests itself
as a lack of microphone bias voltage (VREF_HIZ) on
node 0x17.
With AD1988_6STACK_DIG the maximum bias voltage (VREF_80)
is applied and the headset operates correctly.

Signed-off-by: Tony Vroon <tony@linx.net>
Tested-by: Doug Redlich <pbrigade@nxltech.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>