kernel: mac80211: set the parent of the ath9k gpio_chip

kernel: mac80211: set the parent of the ath9k gpio_chip

This allows gpiolib to re-use ath9k's devicetree node as GPIO
controller.
Example:

ath9k: ath9k@0 {
	#gpio-cells = <2>;
	gpio-controller;
}

Now the ath9k node can be used just like any other GPIO controller:
	gpios = <&ath9k 1 GPIO_ACTIVE_HIGH>;

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>