net/mlx4_en: Do not count LLC/SNAP in MTU calculation

LLC/SNAP 8 bytes should not be added as part of header calculation.
If used, payload will be decreased accordingly. For MTU of 1500
we'll set 1522 instead of 1523.

Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Reviewed-by: Liran Liss <liranl@mellanox.com>
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>