ath9k: Fix get channel default noise floor

Commit 8da58553cc ("ath9k: Use calibrated noise floor value
when available") introduced regression in ath9k_hw_getchan_noise
where per chain nominal noise floor has been taken instead default
for channel.
Revert to original default channel noise floor.

Fixes: 8da58553cc ("ath9k: Use calibrated noise floor value when available")
Reported-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Signed-off-by: Wojciech Dubowik <Wojciech.Dubowik@neratec.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ath/ath9k/calib.c

@@ -72,7 +72,7 @@ static s16 ath9k_hw_get_default_nf(struct ath_hw *ah,
 s16 ath9k_hw_getchan_noise(struct ath_hw *ah, struct ath9k_channel *chan,
 			   s16 nf)
 {
-	s8 noise = ath9k_hw_get_default_nf(ah, chan, 0);
+	s8 noise = ATH_DEFAULT_NOISE_FLOOR;
 
 	if (nf) {
 		s8 delta = nf - ATH9K_NF_CAL_NOISE_THRESH -