regmap: Fix return code for stub regmap_get_device()

We return a pointer, not an int.
Signed-off-by: Mark Brown <broonie@linaro.org>

include/linux/regmap.h

@@ -733,7 +733,7 @@ static inline struct regmap *dev_get_regmap(struct device *dev,
 static inline struct device *regmap_get_device(struct regmap *map)
 {
 	WARN_ONCE(1, "regmap API is disabled");
-	return -EINVAL;
+	return NULL;
 }
 
 #endif