iio: adc: max9611: explicitly cast gain_selectors

After finding a reasonable gain, the function converts the configured
gain to a gain configuration option selector enum max9611_csa_gain.
Make the conversion clearly visible by using an explicit cast. This
also avoids a warning seen with clang:
  drivers/iio/adc/max9611.c:292:16: warning: implicit conversion from
      enumeration type 'enum max9611_conf_ids' to different enumeration
      type 'enum max9611_csa_gain' [-Wenum-conversion]
                        *csa_gain = gain_selectors[i];
                                  ~ ^~~~~~~~~~~~~~~~~

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>