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>