iio: adc: qcom-vadc-common: use fixp_linear_interpolate

Use new function fixp_linear_interpolate() instead of hand-coding the
linear interpolation.
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20201204025509.1075506-5-dmitry.baryshkov@linaro.orgSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/qcom-vadc-common.c

@@ -2,6 +2,7 @@
 #include <linux/bug.h>
 #include <linux/kernel.h>
 #include <linux/bitops.h>
+#include <linux/fixp-arith.h>
 #include <linux/math64.h>
 #include <linux/log2.h>
 #include <linux/err.h>
@@ -368,10 +369,9 @@ static int qcom_vadc_map_voltage_temp(const struct vadc_map_pt *pts,
 	} else {
 		/* result is between search_index and search_index-1 */
 		/* interpolate linearly */
-		*output = (((s32)((pts[i].y - pts[i - 1].y) *
-			(input - pts[i - 1].x)) /
-			(pts[i].x - pts[i - 1].x)) +
-			pts[i - 1].y);
+		*output = fixp_linear_interpolate(pts[i - 1].x, pts[i - 1].y,
+						  pts[i].x, pts[i].y,
+						  input);
 	}
 
 	return 0;