net: mediatek: remove incorrect dma_mask assignment

Device drivers should not mess with the DMA mask directly,
but instead call dma_set_mask() etc if needed.

In case of the mtk_eth_soc driver, the mask already gets set
correctly when the device is created, and setting it again
is against the documented API.

This removes the incorrect setting.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>