net: ethernet: mtk_eth_soc: use WO firmware for MT7981

In order to support wireless offloading on MT7981 we need to load the
appropriate firmware. Recognize MT7981 and load mt7981_wo.bin.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: David S. Miller <davem@davemloft.net>