powerpc/cell: Fix cell iommu after it_page_shift changes

The patch to add it_page_shift incorrectly changed the increment of
uaddr to use it_page_shift, rather then (1 << it_page_shift).

This broke booting on at least some Cell blades, as the iommu was
basically non-functional.

Fixes: 3a553170 ("powerpc/iommu: Add it_page_shift field to determine iommu page size")
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>