stmmac: Don't exit mdio registration when mdio subnode is not found in the DTS

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>