scsi: a4000 - Correct driver unregistration in case of failure

commit 7a192ec3

 ("platform driver: fix
incorrect use of 'platform_bus_type' with 'struct device_driver') turned a
driver_UNregister into platform_driver_REGISTER. Correct this to
platform_driver_UNregister.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>