mlx4_en: Fix not deleted napi structures

Napi structures are being created each time we open a port, but when
the port is closed the napi structure is only disabled but not removed.
This bug caused hang while removing the driver.

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>