octeon: fix PHY name to match MDIO bus name

Commit "d6c25beb: mdio-octeon: use an unique MDIO bus name" changed the
octeon MDIO bus name from "0" to "mdio-octeon-0", change the PHY
formatting logic to account for that name change, so that PHY connection
on this bus succeeds.

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