net: ethernet: aquantia: Call netdev_register after all initialized.

netdev_register should be called when everything is initialized.
Also we should use net_device->reg_state field instead own
"is_ndev_registered" flag to avoid any race.

Signed-off-by: Pavel Belous <pavel.belous@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>