iio: adc: ad7192: fix channel select

Channel configuration doesn't work as expected.
For FIELD_PREP the bit mask is needed and not the bit number.

Fixes: 874bbd1 ("iio: adc: ad7192: Use bitfield access macros")
Signed-off-by: Markus Burri <markus.burri@mt.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250124150703.97848-1-markus.burri@mt.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: Markus Burri

drivers/iio/adc/ad7192.c

@@ -1084,7 +1084,7 @@ static int ad7192_update_scan_mode(struct iio_dev *indio_dev, const unsigned lon
 
 	conf &= ~AD7192_CONF_CHAN_MASK;
 	for_each_set_bit(i, scan_mask, 8)
-		conf |= FIELD_PREP(AD7192_CONF_CHAN_MASK, i);
+		conf |= FIELD_PREP(AD7192_CONF_CHAN_MASK, BIT(i));
 
 	ret = ad_sd_write_reg(&st->sd, AD7192_REG_CONF, 3, conf);
 	if (ret < 0)