x86/vt-d: Fix comment for dma_pte_free_pagetable()

dma_pte_free_pagetable no longer depends on last level ptes
being clear, it clears them itself.  Fix up the comment to
match.

Cc: Jiang Liu <jiang.liu@linux.intel.com>
Suggested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>