PCI/PM: Disable runtime PM of PCIe ports

The runtime PM of PCIe ports turns out to be quite fragile, as in
some cases things work while in some other cases they don't and we
don't seem to have a good way to determine whether or not they are
going to work in advance.

For this reason, avoid enabling runtime PM for PCIe ports by
keeping their runtime PM reference counters always above 0 for the
time being.

When a PCIe port is suspended, it can no longer report events like
hotplug, so hotplug below the port may not work, as in the bug
report below.

[bhelgaas: changelog, stable]
Reference: https://bugzilla.kernel.org/show_bug.cgi?id=53811


Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: stable@vger.kernel.org	# v3.6+