wifi: mt76: mt7996: fix uninitialized variable in parsing txfree

Fix the uninitialized variable warning in mt7996_mac_tx_free.

Fixes: 2461599f ("wifi: mt76: mt7996: get tx_retries and tx_failed from txfree")
Signed-off-by: Yi-Chia Hsieh <yi-chia.hsieh@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7996/mac.c

@@ -1074,7 +1074,7 @@ mt7996_mac_tx_free(struct mt7996_dev *dev, void *data, int len)
 	struct mt76_phy *phy3 = mdev->phys[MT_BAND2];
 	struct mt76_txwi_cache *txwi;
 	struct ieee80211_sta *sta = NULL;
-	struct mt76_wcid *wcid;
+	struct mt76_wcid *wcid = NULL;
 	LIST_HEAD(free_list);
 	struct sk_buff *skb, *tmp;
 	void *end = data + len;