mt76: mt7921: fix possible NULL pointer dereference in mt7921_mac_write_txwi

Fix a possible NULL pointer deference issue in mt7921_mac_write_txwi
routine if vif is NULL.

Fixes: 33920b2b ("mt76: add support for setting mcast rate")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

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

@@ -981,7 +981,7 @@ void mt7921_mac_write_txwi(struct mt7921_dev *dev, __le32 *txwi,
 		mt7921_mac_write_txwi_80211(dev, txwi, skb, key);
 
 	if (txwi[2] & cpu_to_le32(MT_TXD2_FIX_RATE)) {
-		int rateidx = ffs(vif->bss_conf.basic_rates) - 1;
+		int rateidx = vif ? ffs(vif->bss_conf.basic_rates) - 1 : 0;
 		u16 rate, mode;
 
 		/* hardware won't add HTC for mgmt/ctrl frame */