regulator: lp8788-buck: Silence build warning

This driver use id as array index, thus add bounder checking for id.

This patch fixes below build warning:
drivers/regulator/lp8788-buck.c: In function 'lp8788_buck_probe':
drivers/regulator/lp8788-buck.c:490:8: warning: array subscript is above array bounds [-Warray-bounds]
drivers/regulator/lp8788-buck.c:489:63: warning: array subscript is above array bounds [-Warray-bounds]
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NAxel Lin <axel.lin@ingics.com>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>

drivers/regulator/lp8788-buck.c

@@ -474,7 +474,7 @@ static int lp8788_init_dvs(struct platform_device *pdev,
 	u8 default_dvs_mode[] = { LP8788_BUCK1_DVS_I2C, LP8788_BUCK2_DVS_I2C };
 
 	/* no dvs for buck3, 4 */
-	if (id == BUCK3 || id == BUCK4)
+	if (id > BUCK2)
 		return 0;
 
 	/* no dvs platform data, then dvs will be selected by I2C registers */
@@ -505,6 +505,9 @@ static int lp8788_buck_probe(struct platform_device *pdev)
 	struct regulator_dev *rdev;
 	int ret;
 
+	if (id >= LP8788_NUM_BUCKS)
+		return -EINVAL;
+
 	buck = devm_kzalloc(&pdev->dev, sizeof(struct lp8788_buck), GFP_KERNEL);
 	if (!buck)
 		return -ENOMEM;