powerpc/cell: cell_dma_dev_setup_iommu() return the iommu table

Make cell_dma_dev_setup_iommu() return a pointer to the struct iommu_table
(or NULL if no table can be found) rather than putting this pointer into
dev->archdata.dma_data (let the caller do that), and rename this function
to cell_get_iommu_table() to reflect this change.

This will allow us to get the iommu table for a device that doesn't have
the table in the archdata.

Signed-off-by: Mark Nelson <markn@au1.ibm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>