iio: adc: ad7192: 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: 42776c14 ("staging: iio: adc: ad7192: Add system calibration support")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://patch.msgid.link/8aade802afca6a89641e24c1ae1d4b8d82cff58d.1740655250.git.u.kleine-koenig@baylibre.com


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