Fix the following fallthrough warning (on ARM):

drivers/scsi/arm/acornscsi.c:2651:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
           case res_success:
           ^
   drivers/scsi/arm/acornscsi.c:2651:2: note: insert '__attribute__((fallthrough));' to silence this warning
           case res_success:
           ^
           __attribute__((fallthrough));
   drivers/scsi/arm/acornscsi.c:2651:2: note: insert 'break;' to avoid fall-through
           case res_success:
           ^
           break;
Reported-by: Nkernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/202107260355.bF00i5bi-lkp@intel.com/Signed-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>