iio: adc: ad7124: Implement internal calibration at probe time

Use the calibration function provided by the ad_sigma_delta shim to
calibrate all channels at probe time.

For measurements with gain 1 (i.e. if CONFIG_x.PGA = 0) full-scale
calibrations are not supported and the reset default value of the GAIN
register is supposed to be used then.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://patch.msgid.link/20250303114659.1672695-17-u.kleine-koenig@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>