nvme-pci: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/nvme/host/pci.c: In function ‘nvme_timeout’:
drivers/nvme/host/pci.c:1298:12: warning: this statement may fall through [-Wimplicit-fallthrough=]
   shutdown = true;
   ~~~~~~~~~^~~~~~
drivers/nvme/host/pci.c:1299:2: note: here
  case NVME_CTRL_CONNECTING:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/pci.c

@@ -1296,6 +1296,7 @@ static enum blk_eh_timer_return nvme_timeout(struct request *req, bool reserved)
 	switch (dev->ctrl.state) {
 	case NVME_CTRL_DELETING:
 		shutdown = true;
+		/* fall through */
 	case NVME_CTRL_CONNECTING:
 	case NVME_CTRL_RESETTING:
 		dev_warn_ratelimited(dev->ctrl.device,