powerpc: Fix bug in iommu_alloc_coherent causing hang during boot

In commit 8eb6c6e3, Christoph Hellwig
made iommu_alloc_coherent able to do node-local allocations, but
unfortunately got the order of the arguments to alloc_pages_node
wrong.  This fixes it.

Signed-off-by: Paul Mackerras <paulus@samba.org>