ath9k: Allow configuration of LED polarity in platform data.

Some devices running OpenWrt need this and it makes sense to add this
to ath9k_platform_data as the next patches will add a devicetree
(boolean) property for it as well.

Suggested-by: Vittorio Gambaletta <openwrt@vittgam.net>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>