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>