iommu/amd: Make sure dma_ops are set for hotplug devices

There is a bug introduced with commit 27c2127a that causes
devices which are hot unplugged and then hot-replugged to
not have per-device dma_ops set. This causes these devices
to not function correctly. Fixed with this patch.

Cc: stable@vger.kernel.org
Reported-by: Andreas Degert <andreas.degert@googlemail.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>