Use vport rather than physical-port MTU in mlx5en(4).

Set and report vport MTU rather than physical MTU,
The 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.

Based on Linux upstream commit:
cd255efff9baadd654d6160e52d17ae7c568c9d3

Submitted by: Meny Yossefi <menyy@mellanox.com>
MFC after: 1 week
Sponsored by: Mellanox Technologies