sound: Mark sound_setup() as optional

This method in the sound API is optional since some drivers can do this
when probing or as part of SoC init. Mark it as such.
Signed-off-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>

drivers/sound/sound-uclass.c

@@ -16,7 +16,7 @@ int sound_setup(struct udevice *dev)
 	struct sound_ops *ops = sound_get_ops(dev);
 
 	if (!ops->setup)
-		return -ENOSYS;
+		return 0;
 
 	return ops->setup(dev);
 }

include/sound.h

@@ -54,7 +54,7 @@ void sound_create_square_wave(uint sample_rate, unsigned short *data, int size,
 /* Operations for sound */
 struct sound_ops {
 	/**
-	 * setup() - Set up to play a sound
+	 * setup() - Set up to play a sound (optional)
 	 */
 	int (*setup)(struct udevice *dev);