bcm63xx_enet: use platform device id directly for miibus name

Directly use the platform device for generating the miibus name. This
removes the last user of bcm_enet_priv::mac_id and we can remove the
field.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>