dwc_eth_qos: do not register semi-initialized device

We move register_netdev() to the end of dwceqos_probe() to close any
races where the netdev callbacks are called before the initialization
has finished.

Reported-by: Pavel Andrianov <andrianov@ispras.ru>
Signed-off-by: Lars Persson <larper@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>