swiotlb: Fix the type of index

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

Fixes: 26a7e094 ("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>