scsi: libsas: Fix fall-through warning for Clang

Fix the following fallthrough warning (arm64-randconfig with Clang):

drivers/scsi/libsas/sas_discover.c:467:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
Reported-by: Nkernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/60edca25.k00ut905IFBjPyt5%25lkp@intel.com/Signed-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>

drivers/scsi/libsas/sas_discover.c

@@ -461,7 +461,7 @@ static void sas_discover_domain(struct work_struct *work)
 		break;
 #else
 		pr_notice("ATA device seen but CONFIG_SCSI_SAS_ATA=N so cannot attach\n");
-		/* Fall through */
+		fallthrough;
 #endif
 		/* Fall through - only for the #else condition above. */
 	default: