staging: iio: frequency: ad9833: Get frequency value statically

The values from platform data were replaced by statically values.
This was just a intermediate step of taking this driver out of staging and
load data from device tree.

Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>