vdpa/mlx5: Remove mtu field from vdpa net device

No need to save the mtu int the net device struct. We can save it in the
config struct which cannot be modified.

Moreover, move the initialization to. mlx5_vdpa_set_features() callback
is not the right place to put it.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20210909123635.30884-2-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>