atari_scsi: Convert to platform device

Convert atari_scsi to platform device and eliminate scsi_register().

Validate __setup options later on so that module options are checked as well.

Remove the comment about the scsi mid-layer disabling the host irq as it
is no longer true (AFAICT). Also remove the obsolete slow interrupt stuff
(IRQ_TYPE_SLOW == 0 anyway).

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>