net: ethernet: mediatek: fix error handling inside mtk_mdio_init

Return -ENODEV if the MDIO bus is disabled in the device tree.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: John Crispin <john@phrozen.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>