swiotlb: remove a pointless comment

This comments describes an aspect of the map_sg interface that isn't
even exploited by swiotlb.

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>