cpmac: fallback to switch mode if no PHY chip found

If we were unable to detect a PHY on any of the MDIO bus id we tried instead of
bailing out with -ENODEV, assume the MAC is connected to a switch and use MDIO
bus 0. This unbreaks quite a lot of devices out there whose switch cannot be
detected.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>