iio: gts-helper: Fix division loop

The loop based 64bit division may run for a long time when dividend is a
lot bigger than the divider. Replace the division loop by the
div64_u64() which implementation may be significantly faster.

Tested-by: Subhajit Ghosh <subhajit.ghosh@tweaklogic.com>
Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Fixes: 38416c2 ("iio: light: Add gain-time-scale helpers")
Link: https://lore.kernel.org/r/Zcn-6e-0-nh2WcfU@drtxq0yyyyyyyyyyyyyby-3.rev.dnainternet.fi
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: M-Vaittinen

drivers/iio/industrialio-gts-helper.c

@@ -34,24 +34,11 @@
 static int iio_gts_get_gain(const u64 max, const u64 scale)
 {
 	u64 full = max;
-	int tmp = 1;
 
 	if (scale > full || !scale)
 		return -EINVAL;
 
-	if (U64_MAX - full < scale) {
-		/* Risk of overflow */
-		if (full - scale < scale)
-			return 1;
-
-		full -= scale;
-		tmp++;
-	}
-
-	while (full > scale * (u64)tmp)
-		tmp++;
-
-	return tmp;
+	return div64_u64(full, scale);
 }
 
 /**