Second set of IIO fixes for the 3.16 cycle. * A fix for a bug in setting threshold levels within the ad799x driver which prevents correct setting of the thresholds. * In ad7291 fix an case where a ERR_PTR value was returned directly instead of having PTR_ERR applied. Hence it would report success instead of failure. * of_iio_channel_get_by_name returned a non null pointer if it fails and the callee was expecting NULL to indicate failure. Fixed by returning NULL in the error cases.