iio: adc: ad7124: Remove ability to write filter_low_pass_3db_frequency

There are several issues with the function that implements writing to
the filter_low_pass_3db_frequency property:

 - The sinc3 factor should be 0.272 not 0.262 (this is fixed for the
   reading side in the previous patch).
 - For freq > 1 the if condition is always true so the sinc4 filter is
   hardly ever chosen.
 - In the nearly always taken if branch the filter is set to sinc3, but
   the frequency is set for sinc4. (And vice versa in the else branch.)

This is broken enough to justify the claim that there isn't any serious
user. Also it it counter-intuitive that setting the 3db frequency
modifies the sample frequency and the filter type.

So drop the ability to write that property.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/20250317115247.3735016-7-u.kleine-koenig@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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