ALSA: ppc: Constify snd_device_ops definitions

Now we may declare const for snd_device_ops definitions, so let's do
it for optimization.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-16-tiwai@suse.deSigned-off-by: NTakashi Iwai <tiwai@suse.de>

sound/ppc/pmac.c

@@ -1143,7 +1143,7 @@ int snd_pmac_new(struct snd_card *card, struct snd_pmac **chip_return)
 	int i, err;
 	unsigned int irq;
 	unsigned long ctrl_addr, txdma_addr, rxdma_addr;
-	static struct snd_device_ops ops = {
+	static const struct snd_device_ops ops = {
 		.dev_free =	snd_pmac_dev_free,
 	};