iommu/vt-d: Set the right field for Page Walk Snoop

Set the page walk snoop to the right bit, otherwise the domain
id field will be overlapped.

Reported-by: Dave Jiang <dave.jiang@intel.com>
Fixes: 6f7db75e ("iommu/vt-d: Add second level page table interface")
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>