iommu/amd: Print reason for iommu_map_page failure in map_sg

Since there are multiple possible failures in iommu_map_page
it would be useful to know which case is being hit when the
error message is printed in map_sg. While here, fix up checkpatch
complaint about using function name in a string instead of
__func__.

Cc: Joerg Roedel <joro@8bytes.org>
Signed-off-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>