First round of IIO fixes for the 3.19 cycle. * ad799x fix ad7991/ad7995/ad7999 setup as they do not have a configuration register to write to. It is written during the convesion sequence. As such we don't want to write to it at other times. * Fix iio_channel_read utility function to return to ensure it is apparent if the relevant element is not there. This avoids using a wrong value if some channels have the element and others do not.