virtio: mark vring_dma_dev() static

We get 1 warning when building kernel with W=1:
drivers/virtio/virtio_ring.c:170:16: warning: no previous prototype for 'vring_dma_dev' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
so this patch marks this function with 'static'.
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_ring.c

@@ -167,7 +167,7 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
  * making all of the arch DMA ops work on the vring device itself
  * is a mess.  For now, we use the parent device for DMA ops.
  */
-struct device *vring_dma_dev(const struct vring_virtqueue *vq)
+static struct device *vring_dma_dev(const struct vring_virtqueue *vq)
 {
 	return vq->vq.vdev->dev.parent;
 }