iio: adc: ad7124: Fix 3dB filter frequency reading

iio: adc: ad7124: Fix 3dB filter frequency reading

The sinc4 filter has a factor 0.23 between Output Data Rate and f_{3dB}
and for sinc3 the factor is 0.272 according to the data sheets for
ad7124-4 (Rev. E.) and ad7124-8 (Rev. F).

Fixes: cef2760954cf ("iio: adc: ad7124: add 3db filter")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/20250317115247.3735016-6-u.kleine-koenig@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>