iommu/amd: Wait for completion of IOTLB flush in attach_device

To make sure the domain tlb flush completes before the
function returns, explicitly wait for its completion.

Signed-off-by: Filippo Sironi <sironi@amazon.de>
Fixes: 42a49f96 ("amd-iommu: flush domain tlb when attaching a new device")
[joro: Added commit message and fixes tag]
Signed-off-by: Joerg Roedel <jroedel@suse.de>