Originally, most of the platforms using this driver did not define an mdio subnode
in the devicetree. Commit e34d65 ("stmmac: create of compatible mdio bus for stmmac driver")
introduced a backward compatibily issue by using of_mdiobus_register explicitly
with an mdio subnode. This patch fixes the issue by calling the function
mdiobus_register, when mdio subnode is not found. The driver is now compatible
with both modes.

Fixes: e34d6569 ("stmmac: create of compatible mdio bus for stmmac driver")
Signed-off-by: Romain Perier <romain.perier@gmail.com>
Tested-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>