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: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/drivers/sound/sound-uclass.c b/drivers/sound/sound-uclass.c
index 2b83626..0676606 100644
--- a/drivers/sound/sound-uclass.c
+++ b/drivers/sound/sound-uclass.c
@@ -16,7 +16,7 @@
 	struct sound_ops *ops = sound_get_ops(dev);
 
 	if (!ops->setup)
-		return -ENOSYS;
+		return 0;
 
 	return ops->setup(dev);
 }