ASoC: Work around warnings from some build environments

BUG() should be marked as not returning but for at least some
configurations (including some widely deployed compilers) that's either
not happening or being forgotten by the compiler.  Add some extra return
statements to the affected paths.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

sound/soc/codecs/wm8903.c

@@ -392,6 +392,7 @@ static int wm8903_output_event(struct snd_soc_dapm_widget *w,
 		break;
 	default:
 		BUG();
+		return -EINVAL;  /* Spurious warning from some compilers */
 	}
 
 	switch (w->shift) {
@@ -403,6 +404,7 @@ static int wm8903_output_event(struct snd_soc_dapm_widget *w,
 		break;
 	default:
 		BUG();
+		return -EINVAL;  /* Spurious warning from some compilers */
 	}
 
 	if (event & SND_SOC_DAPM_PRE_PMU) {