virtio_ring: fix description of virtqueue_get_buf

The device (not the driver) populates the used ring and includes the len
of how much data was written.

Signed-off-by: Felipe Franciosi <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/virtio/virtio_ring.c

···
  * @vq: the struct virtqueue we're talking about.
  * @len: the length written into the buffer
  *
- * If the driver wrote data into the buffer, @len will be set to the
+ * If the device wrote data into the buffer, @len will be set to the
  * amount written.  This means you don't need to clear the buffer
  * beforehand to ensure there's no data leakage in the case of short
  * writes.