ath9k: fix key allocation error handling for powersave keys

commit 4ef69d03 upstream.

If no keycache slots are available, ath_key_config can return -ENOSPC.
If the key index is not checked for errors, it can lead to logspam that
looks like this: "ath: wiphy0: keyreset: keycache entry 228 out of range"
This can cause follow-up errors if the invalid keycache index gets
used for tx.
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

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

@@ -1793,6 +1793,7 @@ static int ath9k_sta_add(struct ieee80211_hw *hw,
 	struct ath_common *common = ath9k_hw_common(sc->sc_ah);
 	struct ath_node *an = (struct ath_node *) sta->drv_priv;
 	struct ieee80211_key_conf ps_key = { };
+	int key;
 
 	ath_node_attach(sc, sta);
 
@@ -1800,7 +1801,9 @@ static int ath9k_sta_add(struct ieee80211_hw *hw,
 	    vif->type != NL80211_IFTYPE_AP_VLAN)
 		return 0;
 
-	an->ps_key = ath_key_config(common, vif, sta, &ps_key);
+	key = ath_key_config(common, vif, sta, &ps_key);
+	if (key > 0)
+		an->ps_key = key;
 
 	return 0;
 }
@@ -1817,6 +1820,7 @@ static void ath9k_del_ps_key(struct ath_softc *sc,
 	    return;
 
 	ath_key_delete(common, &ps_key);
+	an->ps_key = 0;
 }
 
 static int ath9k_sta_remove(struct ieee80211_hw *hw,