ALSA: mixart: Support PCM sync_stop

The driver invokes snd_pcm_period_elapsed() simply from the threaded
interrupt handler.  Set card->sync_irq for enabling the missing
sync_stop PCM operation.

Link: https://lore.kernel.org/r/20191210063454.31603-51-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/mixart/mixart.c

@@ -1041,6 +1041,7 @@ static int snd_mixart_create(struct mixart_mgr *mgr, struct snd_card *card, int
 	chip->card = card;
 	chip->chip_idx = idx;
 	chip->mgr = mgr;
+	card->sync_irq = mgr->irq;
 
 	if ((err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops)) < 0) {
 		snd_mixart_chip_free(chip);