PCI: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This fixes the following warning (Building: allmodconfig i386):

  drivers/pci/hotplug/ibmphp_res.c: In function ‘update_bridge_ranges’:
  drivers/pci/hotplug/ibmphp_res.c:1943:16: warning: this statement may fall through [-Wimplicit-fallthrough=]
	 function = 0x8;
	 ~~~~~~~~~^~~~~
  drivers/pci/hotplug/ibmphp_res.c:1944:6: note: here
	case PCI_HEADER_TYPE_MULTIBRIDGE:
	^~~~

Link: https://lore.kernel.org/r/20190802012248.GA22622@embeddedor
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>