mt76: fix possible NULL pointer dereferencing in mt76x2_mac_write_txwi()

Verify wcid is not NULL before dereferencing the pointer to initialize
txwi rate/power info

Fixes: 7bc04215 ("mt76: add driver code for MT76x2e")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/mediatek/mt76/mt76x2_mac.c

@@ -186,7 +186,7 @@ void mt76x2_mac_write_txwi(struct mt76x2_dev *dev, struct mt76x2_txwi *txwi,
 	txwi->pktid = 1;
 
 	spin_lock_bh(&dev->mt76.lock);
-	if (rate->idx < 0 || !rate->count) {
+	if (wcid && (rate->idx < 0 || !rate->count)) {
 		txwi->rate = wcid->tx_rate;
 		max_txpwr_adj = wcid->max_txpwr_adj;
 		nss = wcid->tx_rate_nss;