ixp4xx-eth: fix PHY name to match MDIO bus name

Commit 0869b3a4: ixp4xx-eth: use an unique MDIO bus name changed
the MDIO bus name from "0" to "ixp4xx-eth-0", as a result the PHY
name is not longer appropriate and will not match the MDIO bus name
so PHY connection will not succeed, fix that.

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