net: ethernet: mtk_eth_soc: increase DMA ring sizes

256 descriptors is not enough for multi-gigabit traffic under load on
MT7622. Bump it to 512 to improve performance.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>