PCI: Mark expected switch fall-throughs

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

This patch fixes the following warnings:

  drivers/pci/proc.c: In function ‘proc_bus_pci_ioctl’:
  drivers/pci/proc.c:216:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
     if (arch_can_pci_mmap_wc()) {
        ^
  drivers/pci/proc.c:225:2: note: here
    default:
    ^~~~~~~

  drivers/pci/xen-pcifront.c: In function ‘pcifront_backend_changed’:
  drivers/pci/xen-pcifront.c:1105:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
     if (xdev->state == XenbusStateClosed)
        ^
  drivers/pci/xen-pcifront.c:1108:2: note: here
    case XenbusStateClosing:
    ^~~~

Notice that, in this particular case, the /* fall through */
comment is placed at the very bottom of the case statement,
which is what GCC is expecting to find.

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>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>