regulator: Return an error from stubbed regulator_get_exclusive()

The user hasn't got a regulator and shouldn't be mislead into thinking
they have one; really we should probably remove this stub entirely (and
may well before the next merge window).
Signed-off-by: Mark Brown <broonie@kernel.org>

include/linux/regulator/consumer.h

@@ -282,7 +282,7 @@ devm_regulator_get(struct device *dev, const char *id)
 static inline struct regulator *__must_check
 regulator_get_exclusive(struct device *dev, const char *id)
 {
-	return NULL;
+	return ERR_PTR(-ENODEV);
 }
 
 static inline struct regulator *__must_check