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>