ath9k: Add a module parameter to invert LED polarity.

The LED can be active high instead of active low on some hardware.

Add the led_active_high module parameter. It defaults to -1 to obey
platform data as before.

Setting the parameter to 1 or 0 will force the LED respectively
active high or active low.

Cc: <linux-wireless@vger.kernel.org>
Cc: <ath9k-devel@qca.qualcomm.com>
Cc: <ath9k-devel@lists.ath9k.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Vittorio Gambaletta <linuxbugs@vittgam.net>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>