virtio_pci_modern: correct sparse tags for notify

When switching virtio_pci_modern to use a helper for mappings we lost an
__iomem tag. Restore it.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 9e3bb9b7 ("virtio_pci_modern: introduce helper to map vq notify area")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>