ath9k: add an interface for overriding the value of specific GPIO pins

Some devices control antenna settings or other things through GPIO pins
of the wireless interface. Add a debugfs interface for changing those
and keeping them set across card resets.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>