MFC r344920:
Teardown ifnet after stopping port in the mlx4en(4) driver.

mlx4_en_stop_port() calls mlx4_en_put_qp() which can refer the link level
address of the network interface, which in turn will be freed by the
network interface detach function. Make sure the port is stopped
before detaching the network interface.

Sponsored by: Mellanox Technologies