powerpc/dma: Fix dma_iommu_dma_supported compare

The table offset is in entries, each of which imply a dma address of
an IOMMU page.

Also, we should check the device can reach the whole IOMMU table.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>