vdpa/mlx5: Avoid losing link state updates

Current code ignores link state updates if VIRTIO_NET_F_STATUS was not
negotiated. However, link state updates could be received before feature
negotiation was completed , therefore causing link state events to be
lost, possibly leaving the link state down.

Modify the code so link state notifier is registered after DRIVER_OK was
negotiated and carry the registration only if
VIRTIO_NET_F_STATUS was negotiated.  Unregister the notifier when the
device is reset.

Fixes: 033779a7 ("vdpa/mlx5: make MTU/STATUS presence conditional on feature bits")
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Eli Cohen <elic@nvidia.com>
Message-Id: <20230417110343.138319-1-elic@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>