iio: adc: ad7124: Fix comparison of channel configs

iio: adc: ad7124: Fix comparison of channel configs

Checking the binary representation of two structs (of the same type)
for equality doesn't have the same semantic as comparing all members for
equality. The former might find a difference where the latter doesn't in
the presence of padding or when ambiguous types like float or bool are
involved. (Floats typically have different representations for single
values, like -0.0 vs +0.0, or 0.5 * 2² vs 0.25 * 2³. The type bool has
at least 8 bits and the raw values 1 and 2 (probably) both evaluate to
true, but memcmp finds a difference.)

When searching for a channel that already has the configuration we need,
the comparison by member is the one that is needed.

Convert the comparison accordingly to compare the members one after
another. Also add a static_assert guard to (somewhat) ensure that when
struct ad7124_channel_config::config_props is expanded, the comparison
is adapted, too.

This issue is somewhat theoretic, but using memcmp() on a struct is a
bad pattern that is worth fixing.

Fixes: 7b8d045e497a ("iio: adc: ad7124: allow more than 8 channels")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://patch.msgid.link/20250303114659.1672695-13-u.kleine-koenig@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>