of_mdio: fix node leak in of_phy_register_fixed_link error path

Make sure to drop the of_node reference also on failure to parse the
speed property in of_phy_register_fixed_link().

Fixes: 3be2a49e ("of: provide a binding for fixed link PHYs")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>