ath9k: implement .get_antenna and .set_antenna

On MIMO chips this can be used to enable/disable hardware chains, ensuring
that the MCS information is updated accordingly.
On non-MIMO chips with rx diversity (e.g. 9285), this configures the rx
input antenna.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath9k/recv.c b/drivers/net/wireless/ath/ath9k/recv.c
index 9c7f905..8d3e19d 100644
--- a/drivers/net/wireless/ath/ath9k/recv.c
+++ b/drivers/net/wireless/ath/ath9k/recv.c
@@ -1950,7 +1950,7 @@
 			ath_rx_ps(sc, skb);
 		spin_unlock_irqrestore(&sc->sc_pm_lock, flags);
 
-		if (ah->caps.hw_caps & ATH9K_HW_CAP_ANT_DIV_COMB)
+		if ((ah->caps.hw_caps & ATH9K_HW_CAP_ANT_DIV_COMB) && sc->ant_rx == 3)
 			ath_ant_comb_scan(sc, &rs);
 
 		ieee80211_rx(hw, skb);