powerpc/cell: clean up fixed mapping dma_ops initialization

By the time cell_pci_dma_dev_setup calls cell_dma_dev_setup no device can
have the fixed map_ops set yet as it's only set by the set_dma_mask
method.  So move the setup for the fixed case to be only called in that
place instead of indirecting through cell_dma_dev_setup.

Signed-off-by: Christoph Hellwig <hch@lst.de>