ath9k: add noise floor parameter to ath9k_hw_getchan_noise

Add nf parameter to ath9k_hw_getchan_noise() in order to compute NF for EXT
chains with the same scale of noise floor calculated on CTL chains.
ath9k_hw_getchan_noise() will be used in ath_process_fft() for spectral scan on
HT40 channels

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>