net: ethernet: mediatek: fix inconsistency between TXD and the used buffer

Fix inconsistency between the TXD descriptor and the used buffer that
would cause unexpected logic at mtk_tx_unmap() during skb housekeeping.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>