OF: Simplify of_iommu_configure()

OF: Simplify of_iommu_configure()

We no longer have a notion of partially-initialised fwspecs existing,
and we also no longer need to use an iommu_ops pointer to return status
to of_dma_configure(). Clean up the remains of those, which lends itself
to clarifying the logic around the dma_range_map allocation as well.

Acked-by: Rob Herring (Arm) <robh@kernel.org>
Tested-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/61972f88e31a6eda8bf5852f0853951164279a3c.1719919669.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>