iio: adc: ad7124: fix config comparison

iio: adc: ad7124: fix config comparison

The ad7124_find_similar_live_cfg() computes the compare size by
substracting the address of the cfg struct from the address of the live
field. Because the live field is the first field in the struct, the
result is 0.

Also, the memcmp() call is made from the start of the cfg struct, which
includes the live and cfg_slot fields, which are not relevant for the
comparison.

Fix by grouping the relevant fields with struct_group() and use the
size of the group to compute the compare size; make the memcmp() call
from the address of the group.

Fixes: 7b8d045e497a ("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/20240731-ad7124-fix-v1-2-46a76aa4b9be@analog.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>