ALSA: es18xx: 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/es18xx.c

@@ -1665,7 +1665,7 @@ static int snd_es18xx_probe(struct snd_es18xx *chip,
 	return snd_es18xx_initialize(chip, mpu_port, fm_port);
 }
 
-static struct snd_pcm_ops snd_es18xx_playback_ops = {
+static const struct snd_pcm_ops snd_es18xx_playback_ops = {
 	.open =		snd_es18xx_playback_open,
 	.close =	snd_es18xx_playback_close,
 	.ioctl =	snd_pcm_lib_ioctl,
@@ -1676,7 +1676,7 @@ static struct snd_pcm_ops snd_es18xx_playback_ops = {
 	.pointer =	snd_es18xx_playback_pointer,
 };
 
-static struct snd_pcm_ops snd_es18xx_capture_ops = {
+static const struct snd_pcm_ops snd_es18xx_capture_ops = {
 	.open =		snd_es18xx_capture_open,
 	.close =	snd_es18xx_capture_close,
 	.ioctl =	snd_pcm_lib_ioctl,