wifi: mt76: mt7996: fix possible NULL pointer dereference in mt7996_mac_write_txwi()

Fix possible NULL pointer dereference on mvif pointer in
mt7996_mac_write_txwi routine.

Fixes: 15ee62e7 ("wifi: mt76: mt7996: enable BSS_CHANGED_BASIC_RATES support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/2637628a84f42ad6d7b774e706f041d5b45c8cb5.1685269638.git.lorenzo@kernel.org

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

@@ -1004,10 +1004,10 @@ void mt7996_mac_write_txwi(struct mt7996_dev *dev, __le32 *txwi,
 {
 	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
 	struct ieee80211_vif *vif = info->control.vif;
-	struct mt7996_vif *mvif = (struct mt7996_vif *)vif->drv_priv;
 	u8 band_idx = (info->hw_queue & MT_TX_HW_QUEUE_PHY) >> 2;
 	u8 p_fmt, q_idx, omac_idx = 0, wmm_idx = 0;
 	bool is_8023 = info->flags & IEEE80211_TX_CTL_HW_80211_ENCAP;
+	struct mt7996_vif *mvif;
 	u16 tx_count = 15;
 	u32 val;
 	bool beacon = !!(changed & (BSS_CHANGED_BEACON |
@@ -1015,7 +1015,8 @@ void mt7996_mac_write_txwi(struct mt7996_dev *dev, __le32 *txwi,
 	bool inband_disc = !!(changed & (BSS_CHANGED_UNSOL_BCAST_PROBE_RESP |
 					 BSS_CHANGED_FILS_DISCOVERY));
 
-	if (vif) {
+	mvif = vif ? (struct mt7996_vif *)vif->drv_priv : NULL;
+	if (mvif) {
 		omac_idx = mvif->mt76.omac_idx;
 		wmm_idx = mvif->mt76.wmm_idx;
 		band_idx = mvif->mt76.band_idx;
@@ -1081,12 +1082,16 @@ void mt7996_mac_write_txwi(struct mt7996_dev *dev, __le32 *txwi,
 		struct ieee80211_hdr *hdr = (struct ieee80211_hdr *)skb->data;
 		bool mcast = ieee80211_is_data(hdr->frame_control) &&
 			     is_multicast_ether_addr(hdr->addr1);
-		u8 idx = mvif->basic_rates_idx;
+		u8 idx = MT7996_BASIC_RATES_TBL;
 
-		if (mcast && mvif->mcast_rates_idx)
-			idx = mvif->mcast_rates_idx;
-		else if (beacon && mvif->beacon_rates_idx)
-			idx = mvif->beacon_rates_idx;
+		if (mvif) {
+			if (mcast && mvif->mcast_rates_idx)
+				idx = mvif->mcast_rates_idx;
+			else if (beacon && mvif->beacon_rates_idx)
+				idx = mvif->beacon_rates_idx;
+			else
+				idx = mvif->basic_rates_idx;
+		}
 
 		txwi[6] |= cpu_to_le32(FIELD_PREP(MT_TXD6_TX_RATE, idx));
 		txwi[3] |= cpu_to_le32(MT_TXD3_BA_DISABLE);