net/mlx4_en: get rid of private net_device_stats

We simply can use the standard net_device stats.

We do not need to clear fields that are already 0.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>