wifi: mt76: mt7921: reset msta->airtime_ac while clearing up hw value

We should reset mstat->airtime_ac along with clear up the entries in the
hardware WLAN table for the Rx and Rx accumulative airtime. Otherwsie, the
value msta->airtime_ac - [tx, rx]_last may be a negative and that is not
the actual airtime the device took in the last run.
Reported-by: YN Chen <YN.Chen@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7921/main.c

@@ -752,6 +752,7 @@ void mt7921_mac_sta_assoc(struct mt76_dev *mdev, struct ieee80211_vif *vif,
 
 	mt7921_mac_wtbl_update(dev, msta->wcid.idx,
 			       MT_WTBL_UPDATE_ADM_COUNT_CLEAR);
+	memset(msta->airtime_ac, 0, sizeof(msta->airtime_ac));
 
 	mt7921_mcu_sta_update(dev, sta, vif, true, MT76_STA_INFO_STATE_ASSOC);