net/virtio: fix multiple queue enabling

net/virtio: fix multiple queue enabling

When queue number shrinks to 1 from X, the following code stops us
sending the multiple queue ctrl message:

        if (nb_queues > 1) {
                if (virtio_set_multiple_queues(dev, nb_queues) != 0)
                        return -EINVAL;
        }

This ends up with still X queues being enabled, which is obviously
wrong. Fix it by replacing the check with a multiple queue enabled
or not check.

Fixes: 823ad647950a ("virtio: support multiple queues")

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>