of_mdio: fix device reference leak in of_phy_find_device

Make sure to drop the reference taken by bus_find_device() before
returning NULL from of_phy_find_device() when the found device is not a
PHY.

Fixes: 6ed74236 ("of: of_mdio: Ensure mdio device is a PHY")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>