commit 6954cc1f upstream.

Fix null-pointer dereference at probe when the mdio platform device is
missing (e.g. when it has been disabled in DT).
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>