scsi: Centralise ssp frame information units

The xfer_rdy, command, and task frame's iu structures are not available
in <scsi/sas.h>, but only aic94xx driver folder.  Add them to
include/scsi/sas.h

Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>