iio: adc: ad7173: Grab direct mode for calibration

While a calibration is running, better don't make the device do anything
else.

To enforce that, grab direct mode during calibration.

Fixes: 031bdc8a ("iio: adc: ad7173: add calibration support")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://patch.msgid.link/8319fa2dc881c9899d60db4eba7fe8e984716617.1740655250.git.u.kleine-koenig@baylibre.com


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