net: ethernet: mtk_eth_soc: rely on txd_size field in mtk_poll_tx/mtk_poll_rx

This is a preliminary to ad 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>