ALSA: hda: Comment about snd_hdac_bus_update_rirb() and spinlock

The call of snd_hdac_bus_update_rirb() needs the bus->reg_lock
spinlock protection for concurrency.  Comment about it more
explicitly.

Link: https://lore.kernel.org/r/20191213163005.19116-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/hda/hdac_controller.c

@@ -181,6 +181,7 @@ EXPORT_SYMBOL_GPL(snd_hdac_bus_send_cmd);
  * @bus: HD-audio core bus
  *
  * Usually called from interrupt handler.
+ * The caller needs bus->reg_lock spinlock before calling this.
  */
 void snd_hdac_bus_update_rirb(struct hdac_bus *bus)
 {