ASoC: core: Return -ENOTSUPP instead of -EINVAL if mute is not supported

This helps us ignore errors in callers if the operation failed due to not
being available as opposed to an error.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>
diff --git a/sound/soc/soc-core.c b/sound/soc/soc-core.c
index 8a17048..98a4f7a 100644
--- a/sound/soc/soc-core.c
+++ b/sound/soc/soc-core.c
@@ -3031,7 +3031,7 @@
 	if (dai->driver && dai->driver->ops->digital_mute)
 		return dai->driver->ops->digital_mute(dai, mute);
 	else
-		return -EINVAL;
+		return -ENOTSUPP;
 }
 EXPORT_SYMBOL_GPL(snd_soc_dai_digital_mute);