mixer: make all mixer_plugin pointers const

src/mixer/alsa_mixer.c

@@ -237,7 +237,7 @@ alsa_mixer_control(struct mixer_data *data, int cmd, void *arg)
 	return false;
 }
 
-struct mixer_plugin alsa_mixer = {
+const struct mixer_plugin alsa_mixer = {
 	.init = alsa_mixer_init,
 	.finish = alsa_mixer_finish,
 	.configure = alsa_mixer_configure,

src/mixer/oss_mixer.c

@@ -218,7 +218,7 @@ oss_mixer_control(struct mixer_data *data, int cmd, void *arg)
 	return false;
 }
 
-struct mixer_plugin oss_mixer = {
+const struct mixer_plugin oss_mixer = {
 	.init = oss_mixer_init,
 	.finish = oss_mixer_finish,
 	.configure = oss_mixer_configure,

src/mixer_api.c

@@ -21,7 +21,7 @@
 
 #include "mixer_api.h"
 
-void mixer_init(struct mixer *mixer, struct mixer_plugin *plugin)
+void mixer_init(struct mixer *mixer, const struct mixer_plugin *plugin)
 {
 	assert(plugin != NULL);
 	assert(mixer != NULL);

src/mixer_api.h

@@ -25,8 +25,8 @@
  * list of currently implemented mixers
  */
 
-extern struct mixer_plugin alsa_mixer;
-extern struct mixer_plugin oss_mixer;
+extern const struct mixer_plugin alsa_mixer;
+extern const struct mixer_plugin oss_mixer;
 
 struct mixer_data;
 
@@ -65,11 +65,11 @@ struct mixer_plugin {
 };
 
 struct mixer {
-	struct mixer_plugin *plugin;
+	const struct mixer_plugin *plugin;
 	struct mixer_data *data;
 };
 
-void mixer_init(struct mixer *mixer, struct mixer_plugin *plugin);
+void mixer_init(struct mixer *mixer, const struct mixer_plugin *plugin);
 void mixer_finish(struct mixer *mixer);
 void mixer_configure(struct mixer *mixer, const struct config_param *param);
 bool mixer_open(struct mixer *mixer);