mlx4_en: Remove FCS bytes from packet length.

When HW doesn't remove FCS bytes they are reported in the completion
byte count, we don't need to take them to skb.

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