iommu/mediatek: Fix handling of of_count_phandle_with_args result

The function can return negative value so it should be assigned to signed
variable. The patch changes also type of related i variable to make code
more compact and coherent.

The problem has been detected using patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci.
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/mtk_iommu.c

@@ -578,8 +578,7 @@ static int mtk_iommu_probe(struct platform_device *pdev)
 	struct resource         *res;
 	struct component_match  *match = NULL;
 	void                    *protect;
-	unsigned int            i, larb_nr;
-	int                     ret;
+	int                     i, larb_nr, ret;
 
 	data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
 	if (!data)