ALSA: opti9xx: Support PCM sync_stop

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

Link: https://lore.kernel.org/r/20191210063454.31603-46-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/isa/opti9xx/opti92x-ad1848.c

@@ -869,6 +869,7 @@ static int snd_opti9xx_probe(struct snd_card *card)
 	}
 #endif
 	chip->irq = irq;
+	card->sync_irq = chip->irq;
 	strcpy(card->driver, chip->name);
 	sprintf(card->shortname, "OPTi %s", card->driver);
 #if defined(CS4231) || defined(OPTi93X)