mfd: retu: Constify struct regmap_config

The regmap_config struct may be const because it is not modified by the
driver and regmap_init() accepts pointer to const.
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/retu-mfd.c

@@ -222,7 +222,7 @@ static struct regmap_bus retu_bus = {
 	.val_format_endian_default = REGMAP_ENDIAN_NATIVE,
 };
 
-static struct regmap_config retu_config = {
+static const struct regmap_config retu_config = {
 	.reg_bits = 8,
 	.val_bits = 16,
 };