mt76: fix encap offload ethernet type check

The driver needs to check if the format is 802.2 vs 802.3 in order to set
a tx descriptor flag. skb->protocol can't be used, since it may not be properly
initialized for packets coming in from a packet socket.
Fix misdetection by checking the ethertype from the skb data instead
Reported-by: Thibaut VARÈNE <hacks+kernel@slashdirt.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7915/mac.c

@@ -1016,6 +1016,7 @@ mt7915_mac_write_txwi_8023(struct mt7915_dev *dev, __le32 *txwi,
 
 	u8 tid = skb->priority & IEEE80211_QOS_CTL_TID_MASK;
 	u8 fc_type, fc_stype;
+	u16 ethertype;
 	bool wmm = false;
 	u32 val;
 
@@ -1029,7 +1030,8 @@ mt7915_mac_write_txwi_8023(struct mt7915_dev *dev, __le32 *txwi,
 	val = FIELD_PREP(MT_TXD1_HDR_FORMAT, MT_HDR_FORMAT_802_3) |
 	      FIELD_PREP(MT_TXD1_TID, tid);
 
-	if (be16_to_cpu(skb->protocol) >= ETH_P_802_3_MIN)
+	ethertype = get_unaligned_be16(&skb->data[12]);
+	if (ethertype >= ETH_P_802_3_MIN)
 		val |= MT_TXD1_ETH_802_3;
 
 	txwi[1] |= cpu_to_le32(val);

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

@@ -814,6 +814,7 @@ mt7921_mac_write_txwi_8023(struct mt7921_dev *dev, __le32 *txwi,
 {
 	u8 tid = skb->priority & IEEE80211_QOS_CTL_TID_MASK;
 	u8 fc_type, fc_stype;
+	u16 ethertype;
 	bool wmm = false;
 	u32 val;
 
@@ -827,7 +828,8 @@ mt7921_mac_write_txwi_8023(struct mt7921_dev *dev, __le32 *txwi,
 	val = FIELD_PREP(MT_TXD1_HDR_FORMAT, MT_HDR_FORMAT_802_3) |
 	      FIELD_PREP(MT_TXD1_TID, tid);
 
-	if (be16_to_cpu(skb->protocol) >= ETH_P_802_3_MIN)
+	ethertype = get_unaligned_be16(&skb->data[12]);
+	if (ethertype >= ETH_P_802_3_MIN)
 		val |= MT_TXD1_ETH_802_3;
 
 	txwi[1] |= cpu_to_le32(val);