ALSA: usb: 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/20200103081714.9560-11-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer.c

@@ -3407,7 +3407,7 @@ static int create_keep_iface_ctl(struct usb_mixer_interface *mixer)
 int snd_usb_create_mixer(struct snd_usb_audio *chip, int ctrlif,
 			 int ignore_error)
 {
-	static struct snd_device_ops dev_ops = {
+	static const struct snd_device_ops dev_ops = {
 		.dev_free = snd_usb_mixer_dev_free
 	};
 	struct usb_mixer_interface *mixer;