powerpc: dart_iommu: Remove check for controller_ops == NULL case

Now that we have ported the calls to iommu_init_early_dart to always
supply a pci_controller_ops struct, we can safely drop the check.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>