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.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>