[ALSA] snd-aoa: enable dual-edge in GPIOs

Apparently some firmware versions forget enabling the dual-edge bit,
snd-powermac did that too and even OSX does sometimes. This should fix
headphone plug detection on those machines.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>