net/mlx4_en: Fix a race between napi poll function and RX ring cleanup

The RX rings were cleaned while there was still possible RX traffic completion
handling.
Change the sequance of events so that the port is closed and the QPs are being
stopped before RX cleanup.

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>