net: stmmac: make mdio register skips PHY scanning for fixed-link

stmmac_mdio_register() lacks fixed-link consideration and only skip PHY
scanning if it has done DT style PHY discovery. So, for DT or ACPI _DSD
setting of fixed-link, the PHY scanning should not happen.

v2: fix incorrect order related to fwnode that is not caught in non-DT
    platform.

Tested-by: Emilio Riva <emilio.riva@ericsson.com>
Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>