ath9k: add HT40 spectral scan capability

Add spectral scan feature on HT40 channels for ath9k. This patch extends
previous capability added by Simon Wunderlich

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Reviewed-by: Simon Wunderlich <simon.wunderlich@s2003.tu-chemnitz.de>
Tested-by: Simon Wunderlich <simon.wunderlich@s2003.tu-chemnitz.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>