iommu/amd: Finish TLB flush in amd_iommu_unmap()

The function only sends the flush command to the IOMMU(s),
but does not wait for its completion when it returns. Fix
that.

Fixes: 601367d7 ('x86/amd-iommu: Remove iommu_flush_domain function')
Cc: stable@vger.kernel.org # >= 2.6.33
Signed-off-by: Joerg Roedel <jroedel@suse.de>