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.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>