mt76: mt76x02u: update ewma pkt len in mt76x02u_tx_prepare_skb

Update ewma packet length in mt76x02u_tx_prepare_skb as it is
done for pci counterpart in order to properly estimate tx time on
current channel

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>