ath9k: Fix NF calibration for single stream cards

Rather than using the chip ID to read only chain-0 CCA
registers and avoid reading chain-1, use the RX chainmask
instead. There are some 1-stream PCI devices based on AR9287
such as TL-WN751ND. Improper NF calibration might result in
DMA errors/timeouts.
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/ar9002_phy.c

@@ -485,7 +485,7 @@ static void ar9002_hw_do_getnf(struct ath_hw *ah,
 	if (IS_CHAN_HT40(ah->curchan))
 		nfarray[3] = sign_extend32(nf, 8);
 
-	if (AR_SREV_9285(ah) || AR_SREV_9271(ah))
+	if (!(ah->rxchainmask & BIT(1)))
 		return;
 
 	nf = MS(REG_READ(ah, AR_PHY_CH1_CCA), AR9280_PHY_CH1_MINCCA_PWR);