ALSA: mark all struct bus_type as const

Now that the driver core can properly handle constant struct bus_type,
move all of the sound subsystem struct bus_type structures as const,
placing them into read-only memory which can not be modified at runtime.

Note, this fixes a duplicate definition of ac97_bus_type, which somehow
was declared extern in a .h file, and then static as a prototype in a .c
file, and then properly later on in the same .c file.  Amazing that no
compiler warning ever showed up for this.

Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.com>
Cc: Dawei Li <set_pte_at@outlook.com>
Cc: Yu Liao <liaoyu15@huawei.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: linux-sound@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/2023121945-immersion-budget-d0aa@gregkh
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: gregkh

include/sound/ac97_codec.h

@@ -410,7 +410,7 @@ int snd_ac97_pcm_close(struct ac97_pcm *pcm);
 int snd_ac97_pcm_double_rate_rules(struct snd_pcm_runtime *runtime);
 
 /* ad hoc AC97 device driver access */
-extern struct bus_type ac97_bus_type;
+extern const struct bus_type ac97_bus_type;
 
 /* AC97 platform_data adding function */
 static inline void snd_ac97_dev_add_pdata(struct snd_ac97 *ac97, void *data)

include/sound/hdaudio.h

@@ -33,7 +33,7 @@ struct hda_device_id;
 /*
  * exported bus type
  */
-extern struct bus_type snd_hda_bus_type;
+extern const struct bus_type snd_hda_bus_type;
 
 /*
  * generic arrays

sound/ac97/bus.c

@@ -28,8 +28,6 @@ static DEFINE_MUTEX(ac97_controllers_mutex);
 static DEFINE_IDR(ac97_adapter_idr);
 static LIST_HEAD(ac97_controllers);
 
-static struct bus_type ac97_bus_type;
-
 static inline struct ac97_controller*
 to_ac97_controller(struct device *ac97_adapter)
 {

sound/ac97_bus.c

@@ -75,7 +75,7 @@ int snd_ac97_reset(struct snd_ac97 *ac97, bool try_warm, unsigned int id,
 }
 EXPORT_SYMBOL_GPL(snd_ac97_reset);
 
-struct bus_type ac97_bus_type = {
+const struct bus_type ac97_bus_type = {
 	.name		= "ac97",
 };
 

sound/hda/hda_bus_type.c

@@ -76,7 +76,7 @@ static int hda_uevent(const struct device *dev, struct kobj_uevent_env *env)
 	return 0;
 }
 
-struct bus_type snd_hda_bus_type = {
+const struct bus_type snd_hda_bus_type = {
 	.name = "hdaudio",
 	.match = hda_bus_match,
 	.uevent = hda_uevent,