m68k/atari, m68k/sun3: Fix SCSI platform device registration when driver is modular

Fixes: 3ff228af ("atari_scsi: Convert to platform device")
Fixes: 0d31f875 ("sun3_scsi: Convert to platform device")
Reported-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>