iio: afe: rescale: use s64 for temporary scale calculations

All four scaling coefficients can take signed values.
Make tmp a signed 64-bit integer and switch to div_s64() to preserve
signs during 64-bit divisions.

Fixes: 8b74816 ("iio: afe: rescale: new driver")
Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Reviewed-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220108205319.2046348-5-liambeguin@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/afe/iio-rescale.c

@@ -39,7 +39,7 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
 			    int *val, int *val2, long mask)
 {
 	struct rescale *rescale = iio_priv(indio_dev);
-	unsigned long long tmp;
+	s64 tmp;
 	int ret;
 
 	switch (mask) {
@@ -77,10 +77,10 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
 			*val2 = rescale->denominator;
 			return IIO_VAL_FRACTIONAL;
 		case IIO_VAL_FRACTIONAL_LOG2:
-			tmp = *val * 1000000000LL;
-			do_div(tmp, rescale->denominator);
+			tmp = (s64)*val * 1000000000LL;
+			tmp = div_s64(tmp, rescale->denominator);
 			tmp *= rescale->numerator;
-			do_div(tmp, 1000000000LL);
+			tmp = div_s64(tmp, 1000000000LL);
 			*val = tmp;
 			return ret;
 		default: