[ALSA] hda-intel - Fix power-off hang on ASUS P5AD2

The hda-intel driver has a problem at power-off on ASUS P5AD2.
It's caused when the position-buffer is enabled -- most likely a
hardware-specific problem.

This patch adds a quirk to avoid the unnecessary enablement of
position-buffer.

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