powerpc: Further compile fixup for STRICT_MM_TYPECHECKS

A patch of mine was recently committed to fix up STRICT_MM_TYPECHECKS
behaviour on powerpc (f5ea64dc).
However, something which breaks it again seems to have slipped in
afterwards.  So, here's another small fix.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/pci-common.c

@@ -610,7 +610,8 @@ int pci_mmap_legacy_page_range(struct pci_bus *bus,
 	pr_debug(" -> mapping phys %llx\n", (unsigned long long)offset);
 
 	vma->vm_pgoff = offset >> PAGE_SHIFT;
-	vma->vm_page_prot |= _PAGE_NO_CACHE | _PAGE_GUARDED;
+	vma->vm_page_prot = __pgprot(pgprot_val(vma->vm_page_prot)
+				     | _PAGE_NO_CACHE | _PAGE_GUARDED);
 	return remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
 			       vma->vm_end - vma->vm_start,
 			       vma->vm_page_prot);