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: 874bbd12 ("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>