PCI/portdrv: Set PCIE_PORT_SERVICE_HP for Root and Downstream Ports only

It is reported that on some systems pciehp binds to an Upstream Port and
attempts to operate it which causes devices below the Port to disappear
from the bus.

This happens because acpiphp sets dev->is_hotplug_bridge for that Port
(after receiving a Device Check notification on it from the platform
firmware via ACPI) during the enumeration of PCI devices.

get_port_device_capability() sees that dev->is_hotplug_bridge is set and
adds PCIE_PORT_SERVICE_HP to Port services (which allows pciehp to bind to
the Port in question) without consulting the PCIe type, which should be
either Root Port or Downstream Port for the hotplug capability to be
present.

Per PCIe r6.0, sec 7.5.3.2, the Slot Implemented bit is only valid for
Downstream Ports (including Root Ports), and PCIe hotplug depends on the
Slot Capabilities / Control / Status registers.

Make get_port_device_capability() more robust by adding a PCIe type check
to it before adding PCIE_PORT_SERVICE_HP to Port services which helps to
avoid the problem.

[bhelgaas: add spec citation]
Suggested-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/4786090.31r3eYUQgx@kreacher


Reported-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lukas Wunner <lukas@wunner.de>