cpmac: fix PHY name to match MDIO bus name

Commit d1733f07: cpmac: use an unique MDIO bus name changed the MDIO bus
name from "1" to "cpmac-1", this breaks the PHY connection logic because
the PHY name still uses the old bus names "0" and "1", fix that to
always use the mdio bus id instead.

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