iio: cros_ec: Move function description to .c file

To prevent comment rot, move function description to
cros_ec_sensors_core.c.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>