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.

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>