regulator: Fix warning with CONFIG_BUG disabled

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>
diff --git a/drivers/regulator/wm831x-dcdc.c b/drivers/regulator/wm831x-dcdc.c
index 8b0d2c4..06df898 100644
--- a/drivers/regulator/wm831x-dcdc.c
+++ b/drivers/regulator/wm831x-dcdc.c
@@ -120,6 +120,7 @@
 		return REGULATOR_MODE_IDLE;
 	default:
 		BUG();
+		return -EINVAL;
 	}
 }