iio: Fix iio_multiply_value use in iio_read_channel_processed_scale

iio: Fix iio_multiply_value use in iio_read_channel_processed_scale

The function iio_multiply_value returns IIO_VAL_INT (1) on success or a
negative error number on failure, while iio_read_channel_processed_scale
should return an error code or 0. This creates a situation where the
expected result is treated as an error. Fix this by checking the
iio_multiply_value result separately, instead of passing it as a return
value.

Fixes: 05f958d003c9 ("iio: Improve iio_read_channel_processed_scale() precision")
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
Reviewed-by: Hans de Goede <johannes.goede@oss.qualcomm.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>