Make the *_data_direction flags of type enum dma_data_direction from
the generic dma mapping API.  The SCSI_DATA_* defines are kept for
compatiblity and map to the dma mapping API values now.
scsi_to_pci_dma_dir and scsi_to_sbus_dma_dir are kept as stubs -
we define them to nothing as both the PCI and SBUS constants have
the same values as the generic dma mapping ones.  Unfortunately
we can't check anymore whether someone messes this up as the values
are enumes now and not cpp defines.