scsi/atari: Revive "atascsi=" setup option

It was documented in Documentation/m68k/kernel-options.txt and
Documentation/scsi/scsi-parameters.txt, but the implementation was
missing.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Michael Schmitz <schmitzmic@googlemail.com>
Cc: James E.J. Bottomley <JBottomley@parallels.com>
Cc: linux-scsi@vger.kernel.org