scsi: ibmvscsi_tgt: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings (Building: powerpc allyesconfig):

drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c: In function 'ibmvscsis_adapter_info':
drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c:1582:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (connection_broken(vscsi))
      ^
drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c:1584:2: note: here
  default:
  ^~~~~~~
drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c: In function 'ibmvscsis_ping_response':
drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c:2494:16: warning: this statement may fall through [-Wimplicit-fallthrough=]
   vscsi->flags |= CLIENT_FAILED;
drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c:2495:2: note: here
  case H_DROPPED:
  ^~~~
drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c:2496:16: warning: this statement may fall through [-Wimplicit-fallthrough=]
   vscsi->flags |= RESPONSE_Q_DOWN;
drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c:2497:2: note: here
  case H_REMOTE_PARM:
  ^~~~

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Bryant G. Ly <bly@catalogicsoftware.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>