ath9k: add extra GPIO led support

ar9550 or later chips, the AR_GPIO_IN_OUT register only can
control GPIO[0:3]. For the extra GPIO, use standard GPIO calls
instead of WMAC internal registers.

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