iommu: Fix comment for iommu_ops.map_sg

The definition of map_sg was split during a recent addition to iommu_ops.
Put it back together.

Fixes: add02cfd ("iommu: Introduce Interface for IOMMU TLB Flushing")
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>