net/mlx4_en: Call napi_synchronize on stop_port

This is instead of calling the actual implementation of
napi_synchronize, for better encapsulation.

Signed-off-by: Ido Shamay <idos@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>