[POWERPC] pasemi: Fix iommu + 64K PAGE_SIZE bug

The powerpc iommu code was refactored by Linas back in the 2.6.20 time
frame to map 4K pages from the generic code, but I had forgotten to go
back and fix my platform driver before submitting it.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>