swiotlb: use swiotlb_map_page in swiotlb_map_sg_attrs

No need to duplicate the code - map_sg is equivalent to map_page
for each page in the scatterlist.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>