net: arc_emac: Fix memleak in arc_mdio_probe

When devm_gpiod_get_optional() fails, bus should be
freed just like when of_mdiobus_register() fails.

Fixes: 1bddd96c ("net: arc_emac: support the phy reset for emac driver")
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>