net/mlx5e: Use vport MTU rather than physical port MTU

Set and report vport MTU rather than physical MTU,
Driver will set both vport and physical port mtu and will
rely on the query of vport mtu.

SRIOV VFs have to report their MTU to their vport manager (PF),
and this will allow them to work with any MTU they need
without failing the request.

Also for some cases where the PF is not a port owner, PF can
work with MTU less than the physical port mtu if set physical
port mtu didn't take effect.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>