iio: adc: ad7124: set initial ADC mode to idle

iio: adc: ad7124: set initial ADC mode to idle

During setup the st->adc_control is 0, which corresponds to a continuous
conversion mode. The reset value for channel 1 is to enable it. The
combined effect of these two is that the ADC will start conversions for
channel 1 without them being read.
This is not neccessarily a problem, but it is an unexpected behavior.

Set the ADC state to idle during setup to avoid this.

Signed-off-by: Dumitru Ceclan <dumitru.ceclan@analog.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240731-ad7124-fix-v1-4-46a76aa4b9be@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>