iio: Fix iio_read_channel_processed_scale()

The code was checking if (ret) from the processed
channel readout, not smart, we need to check if (ret < 0)
as this will likely be something like IIO_VAL_INT.

Fixes: 635ef601 ("iio: Provide iio_read_channel_processed_scale() API")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210323122705.1326362-1-linus.walleij@linaro.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>