ASoC: mt8192: Report an error if when an invalid sidetone gain is written

Reporting an error on invalid values is optional but helpful to userspace
so do so.

Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230223-asoc-mt8192-quick-fixes-v1-3-9a85f90368e1@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/mediatek/mt8192/mt8192-dai-adda.c

@@ -601,6 +601,8 @@ static int stf_positive_gain_set(struct snd_kcontrol *kcontrol,
 					 POSITIVE_GAIN_MASK_SFT,
 					 (gain_db / 6) << POSITIVE_GAIN_SFT,
 					 &change);
+	} else {
+		return -EINVAL;
 	}
 
 	return change;