iio: adc: ad_sigma_delta: Export ad_sd_calibrate

This patch exports the ad_sd_calibrate function in order to be able to
call it from outside ad_sigma_delta.

There are cases where the option to calibrate one channel at a time is
necessary (ex. system calibration for zero scale and full scale).

Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>