regulator: max8952 - fix max8952_set_voltage

In current implementation, vid is declared as u8,
then "vid == -1" is always false, and "vid >= 0" is always true.
Thus change it to s8.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>

drivers/regulator/max8952.c

@@ -136,7 +136,7 @@ static int max8952_set_voltage(struct regulator_dev *rdev,
 				int min_uV, int max_uV)
 {
 	struct max8952_data *max8952 = rdev_get_drvdata(rdev);
-	u8 vid = -1, i;
+	s8 vid = -1, i;
 
 	if (!gpio_is_valid(max8952->pdata->gpio_vid0) ||
 			!gpio_is_valid(max8952->pdata->gpio_vid0)) {