hidpp10: fix finding the closest dpi (#463)

value and dpi are unsigned so the delta calculations was wrong when
dpi is larger than than value. Casting to int should be fine for the
expected range of values in those variables.

src/hidpp10.c

@@ -643,7 +643,7 @@ hidpp10_get_dpi_mapping(struct hidpp10_device *dev, unsigned int value)
 	min_delta = INT_MAX;
 
 	for (i = 0; i < dev->dpi_count; i++) {
-		delta = abs(value - m[i].dpi);
+		delta = abs((int)value - (int)m[i].dpi);
 		if (delta < min_delta) {
 			result = m[i].raw_value;
 			min_delta = delta;