PCI: Mark expected switch fall-through

There is a plan to build the kernel with -Wimplicit-fallthrough and
these places in the code produced warnings (W=1).  Fix them up.

Signed-off-by: Mathieu Malaterre <malat@debian.org>
[bhelgaas: squash into one patch, drop extra changelog detail]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>