PCI: pciehp: Avoid implicit fallthroughs in switch statements

Per Mika's request, add an explicit break to the last case of switch
statements everywhere in pciehp to be more defensive towards future
amendments.

Per Gustavo's request, mark all non-empty implicit fallthroughs with a
comment to silence warnings triggered by -Wimplicit-fallthrough=2.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Gustavo A. R. Silva <gustavo@embeddedor.com>