net/mlx4_en: Move register_netdev() to the end of initialization function

To avoid a race between the open function and everything that happens after
register_netdev() move it to be the last operation called.

Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>