scsi: libsas: sas_discover: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

Fix the following warning (Building: mtx1_defconfig mips):

drivers/scsi/libsas/sas_discover.c: In function ‘sas_discover_domain’:
./include/linux/printk.h:309:2: warning: this statement may fall through [-Wimplicit-fallthrough=]
  printk(KERN_NOTICE pr_fmt(fmt), ##__VA_ARGS__)
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/scsi/libsas/sas_discover.c:459:3: note: in expansion of macro ‘pr_notice’
   pr_notice("ATA device seen but CONFIG_SCSI_SAS_ATA=N so cannot attach\n");
   ^~~~~~~~~
drivers/scsi/libsas/sas_discover.c:462:2: note: here
  default:
  ^~~~~~~
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

drivers/scsi/libsas/sas_discover.c

@@ -459,6 +459,7 @@ static void sas_discover_domain(struct work_struct *work)
 		pr_notice("ATA device seen but CONFIG_SCSI_SAS_ATA=N so cannot attach\n");
 		/* Fall through */
 #endif
+		/* Fall through - only for the #else condition above. */
 	default:
 		error = -ENXIO;
 		pr_err("unhandled device %d\n", dev->dev_type);