net: ethernet: mediatek: remove superfluous local variable for phy address

remove the unused variable for parsing PHY address
and the related logic for sanity test which would
be all already handled done when of_mdiobus_register
was called

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