iommu/dma: Don't reserve PCI I/O windows

Even if a host controller's CPU-side MMIO windows into PCI I/O space do
happen to leak into PCI memory space such that it might treat them as
peer addresses, trying to reserve the corresponding I/O space addresses
doesn't do anything to help solve that problem. Stop doing a silly thing.

Fixes: fade1ec0

 ("iommu/dma: Avoid PCI host bridge windows")
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>