mt76: fix rssi ewma tracking

The generic EWMA code cannot deal with negative numbers, so convert signal
to a positive number before adding it
Fixes mt76x2 AGC tuning
Signed-off-by: NFelix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mac80211.c

@@ -567,7 +567,9 @@ mt76_check_sta(struct mt76_dev *dev, struct sk_buff *skb)
 
 	sta = container_of((void *) wcid, struct ieee80211_sta, drv_priv);
 
-	ewma_signal_add(&wcid->rssi, status->signal);
+	if (status->signal <= 0)
+		ewma_signal_add(&wcid->rssi, -status->signal);
+
 	wcid->inactive_count = 0;
 
 	if (!test_bit(MT_WCID_FLAG_CHECK_PS, &wcid->flags))

drivers/net/wireless/mediatek/mt76/util.c

@@ -100,7 +100,7 @@ int mt76_get_min_avg_rssi(struct mt76_dev *dev)
 
 			spin_lock(&dev->rx_lock);
 			if (wcid->inactive_count++ < 5)
-				cur_rssi = ewma_signal_read(&wcid->rssi);
+				cur_rssi = -ewma_signal_read(&wcid->rssi);
 			else
 				cur_rssi = 0;
 			spin_unlock(&dev->rx_lock);