ath9k: mark ath_fill_led_pin() static

We get 1 warning about global functions without a declaration
in the ath9k gpio driver when building with W=1:
drivers/net/wireless/ath/ath9k/gpio.c:25:6: warning: no previous prototype for 'ath_fill_led_pin' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is declared
and don't need a declaration, but can be made static.
so this patch marks it 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>