cpmac: fix wrong MDIO bus identifier

This patch fixes the wrong MDIO bus identifier which was
set to 0 unconditionaly, suitable for external switches while
it is actually 1 for PHYs different than external switches
which are autodetected.

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