ath9k: avoid accessing MRC registers on single-chain devices

They are not implemented, and accessing them might trigger errors

Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/ar9003_phy.c

@@ -1172,6 +1172,10 @@ skip_ws_det:
 		 * is_on == 0 means MRC CCK is OFF (more noise imm)
 		 */
 		bool is_on = param ? 1 : 0;
+
+		if (ah->caps.rx_chainmask == 1)
+			break;
+
 		REG_RMW_FIELD(ah, AR_PHY_MRC_CCK_CTRL,
 			      AR_PHY_MRC_CCK_ENABLE, is_on);
 		REG_RMW_FIELD(ah, AR_PHY_MRC_CCK_CTRL,