mt76: mt7921: add flush operation

add flush operation to make sure cfg80211_mlme_deauth is able to flush
the deauthtication frame into air immediately.
Co-developed-by: YN Chen <YN.Chen@mediatek.com>
Signed-off-by: YN Chen <YN.Chen@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

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

@@ -1137,6 +1137,15 @@ static void mt7921_set_rekey_data(struct ieee80211_hw *hw,
 }
 #endif /* CONFIG_PM */
 
+static void mt7921_flush(struct ieee80211_hw *hw, struct ieee80211_vif *vif,
+			 u32 queues, bool drop)
+{
+	struct mt7921_dev *dev = mt7921_hw_dev(hw);
+
+	wait_event_timeout(dev->mt76.tx_wait, !mt76_has_tx_pending(&dev->mphy),
+			   HZ / 2);
+}
+
 const struct ieee80211_ops mt7921_ops = {
 	.tx = mt7921_tx,
 	.start = mt7921_start,
@@ -1175,4 +1184,5 @@ const struct ieee80211_ops mt7921_ops = {
 	.set_wakeup = mt7921_set_wakeup,
 	.set_rekey_data = mt7921_set_rekey_data,
 #endif /* CONFIG_PM */
+	.flush = mt7921_flush,
 };