ncr5380: Update usage documentation

Update kernel parameter documentation for atari_scsi, mac_scsi and
g_NCR5380 drivers. Remove duplication.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>