[PATCH] powerpc: fix cell iommu setup

A small bug crept in the iommu driver when we made it more
generic. This patch is needed for boards that have a dma
window that does not start at bus address zero.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>