mac80211: ath9k: add GPIO mask dts property

mac80211: ath9k: add GPIO mask dts property

This patch adds 'qca,gpio-mask=<u32>' device tree property to ath9k node.
This optional setting is a hack and should only be used in very special
(and rare) cases when a button or LED is wired to a GPIO pin normally
masked out (due to being one-way etc). Netgear WNDR4300 v2 is one such
example - it uses GPI9 for RFKILL.

See ath9k/reg.h *_GPIO_MASK constants.

Use with caution and expect to see stream of kernel warnings if wrong
mask value is provided.

Signed-off-by: Michal Cieslakiewicz <michal.cieslakiewicz@wp.pl>