iommu/arm-smmu: Check that iommu_fwspecs are ours

We seem to have forgotten to check that iommu_fwspecs actually belong to
us before we go ahead and dereference their private data. Oops.

Change-Id: I3ca72c2441a08f8600cab62561a0b9b97499f94d
Fixes: 021bb842 ("iommu/arm-smmu: Wire up generic configuration support")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Git-commit: 3c117b54
Git-repo: https://source.codeaurora.org/quic/la/kernel/msm-4.4


Signed-off-by: Patrick Daly <pdaly@codeaurora.org>