virtio-iommu: Remove useless !sdev check in virtio_iommu_probe()

The code already checks iommu_mr is not NULL so there is no
need to check container_of() is not NULL. Remove the check.

Fixes: CID 1523901
Fixes: 09b4c3d6 ("virtio-iommu: Record whether a probe request has
been issued")
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reported-by: Coverity (CID 1523901)
Message-Id: <20231109170715.259520-1-eric.auger@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>