net: of_mdio: fix behavior on missing phy device

of_mdiobus_register creates a phy_device even if get_phy_device failed
to create it previously. This causes indefinite polling on non-existent
PHYs. This fix makes of_mdio_register rely on get_phy_device to
properly create the device or fail otherwise.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>