iommu/of: Fix probe-deferral

Whilst iommu_probe_device() does check for non-NULL ops as the previous
code did, it does not do so in the same order relative to the other
checks, and as a result means that -EPROBE_DEFER returned by of_xlate()
(plus any real error condition too) gets overwritten with -EINVAL and
leads to various misbehaviour.

Reinstate the original logic, but without implicitly relying on ops
being set to infer !err as the initial condition (now that the validity
of ops for its own sake is checked elsewhere).

Fixes: 641fb0ef

 ("iommu/of: Don't call iommu_ops->add_device directly")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>