ALSA: midi: constify snd_rawmidi_global_ops structures

The snd_rawmidi_global_ops structures are never modified, so declare them
as const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/include/sound/rawmidi.h b/include/sound/rawmidi.h
index f6cbef7..fdabbb4 100644
--- a/include/sound/rawmidi.h
+++ b/include/sound/rawmidi.h
@@ -130,7 +130,7 @@
 	int ossreg;
 #endif
 
-	struct snd_rawmidi_global_ops *ops;
+	const struct snd_rawmidi_global_ops *ops;
 
 	struct snd_rawmidi_str streams[2];