net: cpsw: fix null dereference at probe

Fix null-pointer dereference at probe when the mdio platform device is
missing (e.g. when it has been disabled in DT).

Cc: stable <stable@vger.kernel.org>	# v3.8
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>