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: NJaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20210523090920.15345-2-tiwai@suse.deSigned-off-by: NTakashi Iwai <tiwai@suse.de>

include/sound/core.h

@@ -142,12 +142,12 @@ struct snd_card {
 #ifdef CONFIG_PM
 static inline unsigned int snd_power_get_state(struct snd_card *card)
 {
-	return card->power_state;
+	return READ_ONCE(card->power_state);
 }
 
 static inline void snd_power_change_state(struct snd_card *card, unsigned int state)
 {
-	card->power_state = state;
+	WRITE_ONCE(card->power_state, state);
 	wake_up(&card->power_sleep);
 }