iio: adc: ad7791: implement IIO_CHAN_INFO_SAMP_FREQ

Now that the old read/write frequency sysfs attrs have been removed, we
have a clean slate to implement IIO_CHAN_INFO_SAMP_FREQ.

This driver also pre-dates IIO_CHAN_INFO_SAMP_FREQ, and this change
implements this behavior.

The `ad7791_write_raw` would have overlapped quite a bit with the old
read/write frequency functions, making things a bit harder to follow.

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

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>