regulator: Staticise of_get_fixed_voltage_config()

It is not used outside this driver so no need to make the symbol global.
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/regulator/fixed.c

@@ -51,7 +51,8 @@ struct fixed_voltage_data {
  * tree node, returns a pointer to the populated structure of NULL if memory
  * alloc fails.
  */
-struct fixed_voltage_config *of_get_fixed_voltage_config(struct device *dev)
+static struct fixed_voltage_config *
+of_get_fixed_voltage_config(struct device *dev)
 {
 	struct fixed_voltage_config *config;
 	struct device_node *np = dev->of_node;