bcm63xx-enet: fix PHY name to match MDIO bus name

Commit 3e617506: bcm63xx_enet: use an unique MDIO bus name introduced
a regression in the PHY connection logic, since the PHY name was formatted
to expect the bus name to be "0" or "1", whereas it is now "bcm63xx-enet-0"
or "bcm63xx-enet-1".

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