mac_scsi: Cleanup PDMA code

Fix whitespace, remove pointless volatile qualifiers and improve code style
by use of INPUT_DATA_REG and OUTPUT_DATA_REG macros.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>