ALSA: es1688: constify snd_pcm_ops structures

snd_pcm_ops are not supposed to change at runtime. All functions
working with snd_pcm_ops provided by <sound/pcm.h> work with
const snd_pcm_ops. So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/isa/es1688/es1688_lib.c

@@ -706,7 +706,7 @@ exit:
 	return err;
 }
 
-static struct snd_pcm_ops snd_es1688_playback_ops = {
+static const struct snd_pcm_ops snd_es1688_playback_ops = {
 	.open =			snd_es1688_playback_open,
 	.close =		snd_es1688_playback_close,
 	.ioctl =		snd_es1688_ioctl,
@@ -717,7 +717,7 @@ static struct snd_pcm_ops snd_es1688_playback_ops = {
 	.pointer =		snd_es1688_playback_pointer,
 };
 
-static struct snd_pcm_ops snd_es1688_capture_ops = {
+static const struct snd_pcm_ops snd_es1688_capture_ops = {
 	.open =			snd_es1688_capture_open,
 	.close =		snd_es1688_capture_close,
 	.ioctl =		snd_es1688_ioctl,