virtio: Fix comment typo 'CONFIG_S_FAILED'

Without the VIRTIO_ prefix CONFIG_S_FAILED looks like a Kconfig macro.
So use that prefix here too.
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/virtio.h

@@ -78,7 +78,7 @@ bool virtqueue_is_broken(struct virtqueue *vq);
 /**
  * virtio_device - representation of a device using virtio
  * @index: unique position on the virtio bus
- * @failed: saved value for CONFIG_S_FAILED bit (for restore)
+ * @failed: saved value for VIRTIO_CONFIG_S_FAILED bit (for restore)
  * @config_enabled: configuration change reporting enabled
  * @config_change_pending: configuration change reported while disabled
  * @config_lock: protects configuration change reporting