atari_scsi, sun3_scsi: Remove global Scsi_Host pointer

This refactoring removes two global Scsi_Host pointers. This
improves consistency with other ncr5380 drivers. Adopting the same
conventions as the other drivers makes them easier to read.

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