net/mlx4_en: set correct MTU in SRIOV

When setting MTU in SRIOV mode add ETH, VLAN and FCS header length
to the maximum MTU obtained from QUERY_DEV_CAP.

Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>