iio: adc: ad7192: Improve f_order computation

iio: adc: ad7192: Improve f_order computation

Instead of using the f_order member of ad7192_state, a function that
computes the f_order coefficient makes more sense. This coefficient is a
function of the sinc filter and chop filter states.

Remove f_order member of ad7192_state structure. Instead use
ad7192_compute_f_order function to compute the f_order coefficient
according to the sinc filter and chop filter states passed as
parameters.

Add ad7192_get_f_order function that returns the current f_order
coefficient of the device.

Add ad7192_compute_f_adc function that computes the f_adc value
according to the sinc filter and chop filter states passed as
parameters.

Add ad7192_get_f_adc function that returns the current f_adc value of
the device.

Signed-off-by: Alisa-Dariana Roman <alisa.roman@analog.com>
Link: https://lore.kernel.org/r/20230924215148.102491-3-alisadariana@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>