fec: fix PHY name to match fixed MDIO bus name

Commit "391420f7: fec: use an unique MDIO bus name" first modified
the MDIO bus name to include the platform name, then in commit
"a7ed07d5: net: fec: correct phy_name buffer length when init phy_name"
the PHY name formatting was fixed in the case the PHY matches a PHY
driver.

The FEC driver however, also handles the case where we want to attach
to the fixed MDIO bus name, which was previously named "0", and now
"fixed-0". Change the PHY formatting logic to account for that.

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