regulator: stubs: Make stub regulator_get_voltage() return an error

Returning 0 isn't useful, it's not even meaningful if there is a real
regulator there.

Reported-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

include/linux/regulator/consumer.h

@@ -291,7 +291,7 @@ static inline int regulator_set_voltage(struct regulator *regulator,
 
 static inline int regulator_get_voltage(struct regulator *regulator)
 {
-	return 0;
+	return -EINVAL;
 }
 
 static inline int regulator_set_current_limit(struct regulator *regulator,