ALSA: Constify the snd_pcm_substream struct ops field

The ops field of the snd_pcm_substream struct is never modified inside the ALSA
core. Making it const allows drivers to declare their snd_pcm_ops struct as
const.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
index 41b3dfe..82bb029 100644
--- a/sound/core/pcm_lib.c
+++ b/sound/core/pcm_lib.c
@@ -568,7 +568,8 @@
  *
  * Sets the given PCM operators to the pcm instance.
  */
-void snd_pcm_set_ops(struct snd_pcm *pcm, int direction, struct snd_pcm_ops *ops)
+void snd_pcm_set_ops(struct snd_pcm *pcm, int direction,
+		     const struct snd_pcm_ops *ops)
 {
 	struct snd_pcm_str *stream = &pcm->streams[direction];
 	struct snd_pcm_substream *substream;