swiotlb: Fix the type of index

[ Upstream commit 95b079d ]

Fix the type of index from unsigned int to int since find_slots() might
return -1.

Fixes: 26a7e09 ("swiotlb: refactor swiotlb_tbl_map_single")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Claire Chang <tientzu@chromium.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

kernel/dma/swiotlb.c

@@ -608,7 +608,8 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
 		enum dma_data_direction dir, unsigned long attrs)
 {
 	unsigned int offset = swiotlb_align_offset(dev, orig_addr);
-	unsigned int index, i;
+	unsigned int i;
+	int index;
 	phys_addr_t tlb_addr;
 
 	if (no_iotlb_memory)