bcm63xx_enet: do not rely on probe order

Do not rely on the shared device being probed before the enet(sw)
devices. This makes it easier to eventually move out the shared
device as a dma controller driver (what it should be).

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