ath9k: fix/add bits for spectral scanning

Updates for spectral scanning register modification for AR93x:
 * fix bits for spectral_scan_count
 * add bits for new parameters

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>