iio: cros_ec: Use Hertz as unit for sampling frequency

To be compliant with other sensors, set and get sensor sampling
frequency in Hz, not mHz.

Fixes: ae7b02ad ("iio: common: cros_ec_sensors: Expose cros_ec_sensors frequency range via iio sysfs")
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>