iio: adc: ad7124: support fractional sampling_frequency

iio: adc: ad7124: support fractional sampling_frequency

Modify the attribute read/write functions for sampling_frequency and
filter_low_pass_3db_frequency to return fractional values.

These ADCs support output data rates in the single digits, so being
able to specify fractional values is necessary to use all possible
sampling frequencies.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>