iio: ad7793: implement IIO_CHAN_INFO_SAMP_FREQ

This commit is a follow-up to changes made to ad_sigma_delta.h
in staging: iio: ad7192: implement IIO_CHAN_INFO_SAMP_FREQ
which broke ad7793 as it was not altered to match those changes.

This driver predates the availability of IIO_CHAN_INFO_SAMP_FREQ
attribute wherein usage has some advantages like it can be accessed by
in-kernel consumers as well as reduces the code size.

Therefore, use IIO_CHAN_INFO_SAMP_FREQ to implement the
sampling_frequency attribute instead of using IIO_DEV_ATTR_SAMP_FREQ()
macro.

Move code from the functions associated with IIO_DEV_ATTR_SAMP_FREQ()
into respective read and write hooks with the mask set to
IIO_CHAN_INFO_SAMP_FREQ.

Fixes: a13e831f ("staging: iio: ad7192: implement IIO_CHAN_INFO_SAMP_FREQ")

Signed-off-by: Michael Nosthoff <committed@heine.so>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>