net/mlx4_en: Pass user MTU value to Firmware at set port command

When starting the port, driver will inform Firmware about the actual MTU
which does not include implicit headers, such as FCS or VLAN tags.

Signed-off-by: Shaker Daibes <shakerd@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>