staging: iio: ad7780: add filter reading to ad778x

This patch adds the new feature of reading the filter odr value for
ad778x chips. This value is stored in the chip's state struct whenever a
read or write call is performed on the chip's driver.

This feature requires sharing SAMP_FREQ. Since the ad717x does not have
a filter option, the driver only shares the relevant info mask for the
ad778x family.

Signed-off-by: Renato Lui Geh <renatogeh@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>