[atheros] Restore gpio buttons functionality broken in AA

Backfire had working gpio buttons on atheros, but it's broken in AA,
presumably due to gpiolib changes  With the following changes hotplug
button events are properly generated again in trunk.  This patch should
also be applied to the AA branch, as it's a regression from Backfire.

Signed-off-by: Karl Palsson <karlp@remake.is>