iio: cros_ec: Allow enabling/disabling calibration mode

iio: cros_ec: Allow enabling/disabling calibration mode

'calibrate' was a one-shot event sent to the sensor to calibrate itself.
It is used on Bosch sensors (BMI160, BMA254).
Light sensors work differently: They are first put in calibration mode,
tests are run to collect information and calculate the calibration
values to apply. Once done, the sensors are put back in normal mode.

Accept boolean true and false (not just true) to enter/exit calibration
state.

Check "echo 0 > calibrate" is supported.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Nick Vaccaro <nvaccaro@google.com>
Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>