ALSA: core: Use READ_ONCE() / WRITE_ONCE() for power state change

We need proper barriers to handle the power state change of the card
from different CPUs.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20210523090920.15345-2-tiwai@suse.de


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