The cfg pointer is set before reading the channel number that the
configuration should point to. This causes configurations to be shifted
by one channel.
For example setting bipolar to the first channel defined in the DT will
cause bipolar mode to be active on the second defined channel.

Fix by moving the cfg pointer setting after reading the channel number.

Fixes: 7b8d045e ("iio: adc: ad7124: allow more than 8 channels")
Signed-off-by: Dumitru Ceclan <dumitru.ceclan@analog.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240806085133.114547-1-dumitru.ceclan@analog.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>