x86, iommu: Update header comments with appropriate naming

The header comments diverged a bit from the implementation. Lets
re-sync them.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
LKML-Reference: <1286564028-2352-3-git-send-email-konrad.wilk@oracle.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>