ALSA: rawmidi: Add const to snd_rawmidi_ops

Make snd_rawmidi_substream.ops to be a const pointer to be safer and
allow more optimization.  The patches to constify each rawmidi ops
will follow.

Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/include/sound/rawmidi.h b/include/sound/rawmidi.h
index f730b91..492a3ca 100644
--- a/include/sound/rawmidi.h
+++ b/include/sound/rawmidi.h
@@ -103,7 +103,7 @@
 	struct snd_rawmidi_runtime *runtime;
 	struct pid *pid;
 	/* hardware layer */
-	struct snd_rawmidi_ops *ops;
+	const struct snd_rawmidi_ops *ops;
 };
 
 struct snd_rawmidi_file {
@@ -155,7 +155,7 @@
 		    int output_count, int input_count,
 		    struct snd_rawmidi **rmidi);
 void snd_rawmidi_set_ops(struct snd_rawmidi *rmidi, int stream,
-			 struct snd_rawmidi_ops *ops);
+			 const struct snd_rawmidi_ops *ops);
 
 /* callbacks */