ia64: IOMMU passthrough mode shouldn't trigger swiotlb init

Since commit 19943b0e

 ('intel-iommu:
Unify hardware and software passthrough support'), hardware passthrough
mode will do the same as software passthrough mode was doing -- it'll
still use the IOMMU normally for devices which can't address all of
memory. This means that we don't need to bother with swiotlb.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>