iommu: Check for iommu_ops == NULL in iommu_probe_device()

This check needs to be there and got lost at some point
during development. Add it again.

Fixes: 641fb0ef

 ('iommu/of: Don't call iommu_ops->add_device directly')
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reported-by: kernelci.org bot <bot@kernelci.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>