ALSA: isa: 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/20200103081714.9560-8-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/isa/es1688/es1688_lib.c b/sound/isa/es1688/es1688_lib.c
index 632e867..99a833f 100644
--- a/sound/isa/es1688/es1688_lib.c
+++ b/sound/isa/es1688/es1688_lib.c
@@ -612,7 +612,7 @@ int snd_es1688_create(struct snd_card *card,
 		      int dma8,
 		      unsigned short hardware)
 {
-	static struct snd_device_ops ops = {
+	static const struct snd_device_ops ops = {
 		.dev_free =	snd_es1688_dev_free,
 	};