caif_virtio: switch to netif_napi_add_weight()

caif_virtio uses a custom napi weight, switch to the new
API for setting custom weights.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>