ath9k: make GPIO API to support both of WMAC and SOC

commit 61b559de ("ath9k: add extra GPIO led support")
added ath9k to support access SOC's GPIOs, but implemented
in a separated API: ath9k_hw_request_gpio().

So this patch make the APIs more common, to support both
of WMAC and SOC GPIOs. The new APIs as below,

void ath9k_hw_gpio_request_in();
void ath9k_hw_gpio_request_out();
void ath9k_hw_gpio_free();

NOTE, the BSP of the SOC chips(AR9340, AR9531, AR9550, AR9561)
should set the corresponding MUX registers correctly.

Signed-off-by: Miaoqing Pan <miaoqing@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>