iio: light: stk3310: Constify regmap_config

stk3310_regmap_config is not modified and can be made const to allow the
compiler to put it in read-only memory.

Before:
   text    data     bss     dec     hex filename
  16027    5424     128   21579    544b drivers/iio/light/stk3310.o

After:
   text    data     bss     dec     hex filename
  16347    5104     128   21579    544b drivers/iio/light/stk3310.o
Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/light/stk3310.c

@@ -487,7 +487,7 @@ static bool stk3310_is_volatile_reg(struct device *dev, unsigned int reg)
 	}
 }
 
-static struct regmap_config stk3310_regmap_config = {
+static const struct regmap_config stk3310_regmap_config = {
 	.name = STK3310_REGMAP_NAME,
 	.reg_bits = 8,
 	.val_bits = 8,