iio: adc: ad7173: support changing filter type

iio: adc: ad7173: support changing filter type

Add support for changing the filter type to the ad7173 driver.

This family of chips by default uses a sinc5+sinc1 filter. There are
also optional post-filters that can be added in this configuration for
various 50/60Hz rejection purposes. The sinc3 filter doesn't have any
post-filters and handles the output data rate (ODR) a bit differently.
Here, we've opted to use SINC3_MAPx to get the maximum possible
sampling frequencies with the SINC3 filter.

Adding support consists of adding the filter_type and
filter_type_available attributes, making the sampling_frequency
attribute aware of the filter type, and programming the filter
parameters when we configure the channel of the ADC for reading
a sample.

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>