[POWERPC] Use it_offset not pte_offset in cell IOMMU code

The cell IOMMU tce build and free routines use pte_offset to convert
the index passed from the generic IOMMU code into a page table offset.

This takes into account the SPIDER_DMA_OFFSET which sets the top bit
of every DMA address.

However it doesn't cater for the IOMMU window starting at a non-zero
address, as the base of the window is not incorporated into pte_offset
at all.

As it turns out tbl->it_offset already contains the value we need, it
takes into account the base of the window and also pte_offset. So use
it instead!
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/powerpc/platforms/cell/iommu.c

@@ -200,7 +200,7 @@ static void tce_build_cell(struct iommu_table *tbl, long index, long npages,
 		(window->ioid & IOPTE_IOID_Mask);
 #endif
 
-	io_pte = (unsigned long *)tbl->it_base + (index - window->pte_offset);
+	io_pte = (unsigned long *)tbl->it_base + (index - tbl->it_offset);
 
 	for (i = 0; i < npages; i++, uaddr += IOMMU_PAGE_SIZE)
 		io_pte[i] = base_pte | (__pa(uaddr) & IOPTE_RPN_Mask);
@@ -232,7 +232,7 @@ static void tce_free_cell(struct iommu_table *tbl, long index, long npages)
 		| (window->ioid & IOPTE_IOID_Mask);
 #endif
 
-	io_pte = (unsigned long *)tbl->it_base + (index - window->pte_offset);
+	io_pte = (unsigned long *)tbl->it_base + (index - tbl->it_offset);
 
 	for (i = 0; i < npages; i++)
 		io_pte[i] = pte;