net: ethernet: mtk_eth_soc: rely on txd_size in mtk_tx_alloc/mtk_tx_clean

This is a preliminary patch to add mt7986 ethernet support.

Tested-by: Sam Shih <sam.shih@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>