mt76: fix possible NULL pointer dereference in mt76_tx

Even if this is not a real issue since mt76_tx is never run with wcid set
to NULL, fix a theoretical NULL pointer dereference in mt76_tx routine

Fixes: db9f11d3433f7 ("mt76: store wcid tx rate info in one u32 reduce locking")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

tx.c

@@ -306,7 +306,7 @@ mt76_tx(struct mt76_phy *phy, struct ieee80211_sta *sta,
 		skb_set_queue_mapping(skb, qid);
 	}
 
-	if (!(wcid->tx_info & MT_WCID_TX_INFO_SET))
+	if (wcid && !(wcid->tx_info & MT_WCID_TX_INFO_SET))
 		ieee80211_get_tx_rates(info->control.vif, sta, skb,
 				       info->control.rates, 1);