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>