net: ethernet: ti: cpsw: fix mdio device reference leak

Make sure to drop the reference taken by of_find_device_by_node() when
looking up an mdio device from a phy_id property during probe.

Fixes: 549985ee ("cpsw: simplify the setup of the register
pointers")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>