net: ethernet: mtk_eth_soc: rely on rxd_size field in mtk_rx_alloc/mtk_rx_clean

Remove mtk_rx_dma structure layout dependency in mtk_rx_alloc/mtk_rx_clean.
Initialize to 0 rxd3 and rxd4 in mtk_rx_alloc.
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>