net: ethernet: mediatek: get the chip id by ETHDMASYS registers

The driver gets the chip id by ETHSYS_CHIPID0_3/ETHSYS_CHIPID4_7 registers
in mtk_probe().

Signed-off-by: Nelson Chang <nelson.chang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>