wifi: mt76: mt7615: fix possible race in mt7615_mac_sta_poll

Grab sta_poll_lock spinlock in mt7615_mac_sta_poll routine in order to
avoid possible races with mt7615_mac_add_txs() or mt7615_mac_fill_rx()
removing msta pointer from sta_poll_list.

Fixes: a621372a ("mt76: mt7615: rework mt7615_mac_sta_poll for usb code")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/48b23404b759de4f1db2ef85975c72a4aeb1097c.1684938695.git.lorenzo@kernel.org

drivers/net/wireless/mediatek/mt76/mt7615/mac.c

@@ -914,7 +914,10 @@ void mt7615_mac_sta_poll(struct mt7615_dev *dev)
 
 		msta = list_first_entry(&sta_poll_list, struct mt7615_sta,
 					poll_list);
+
+		spin_lock_bh(&dev->sta_poll_lock);
 		list_del_init(&msta->poll_list);
+		spin_unlock_bh(&dev->sta_poll_lock);
 
 		addr = mt7615_mac_wtbl_addr(dev, msta->wcid.idx) + 19 * 4;