net: cpsw: fix null dereference at probe

commit 6954cc1f238199e971ec905c5cc87120806ac981 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>