scsi: acornscsi: Fix fall-through warning for clang

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: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/202107260355.bF00i5bi-lkp@intel.com/Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/scsi/arm/acornscsi.c

@@ -2642,6 +2642,7 @@ int acornscsi_abort(struct scsi_cmnd *SCpnt)
 //#endif
 		clear_bit(SCpnt->device->id * 8 +
 			  (u8)(SCpnt->device->lun & 0x7), host->busyluns);
+		fallthrough;
 
 	/*
 	 * We found the command, and cleared it out.  Either