PCI: Don't restrict MPS for slots below Root Ports

When booting with "pci=pcie_bus_safe", we previously limited the
fabric MPS to 128 when we found:

  (1) A hotplug-capable Downstream Port ("dev->is_hotplug_bridge &&
      pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT"), or

  (2) A hotplug-capable Root Port with a slot that was either empty or
      contained a multi-function device ("dev->is_hotplug_bridge &&
      !list_is_singular(&dev->bus->devices)")

Part (1) is valid, but part (2) is not.

After a hot-add in the slot below a Root Port, we can reconfigure all
MPS values in the fabric below the Root Port because the new device is
the only thing below the Root Port and there are no active drivers.
Therefore, there's no reason to limit the MPS for Root Ports, no
matter what's in the slot.

Test info:

    -+-[0000:40]-+-07.0-[0000:46]--+-00.0  Intel 82576 NIC
                                   \-00.1  Intel 82576 NIC

    0000:40:07.0 Root Port bridge to [bus 46] (MPS supported=256)
    0000:46:00.0 Endpoint                     (MPS supported=512)
    0000:46:00.1 Endpoint                     (MPS supported=512)

    # echo 0 > /sys/bus/pci/slots/7/power
    # echo 1 > /sys/bus/pci/slots/7/power
    pcieport 0000:40:07.0: PCI-E Max Payload Size set to 256/ 256 (was 256)
    pci 0000:46:00.0:      PCI-E Max Payload Size set to 256/ 512 (was 128)
    pci 0000:46:00.1:      PCI-E Max Payload Size set to 256/ 512 (was 128)

Before this change, we set MPS to 128 for the Root Port and both NICs
because the slot contained a multi-function device and

    dev->is_hotplug_bridge && !list_is_singular(&dev->bus->devices)

was true.  After this change, we set it to 256.

[bhelgaas: changelog, comments, split out upstream bridge check]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Jon Mason <jdmason@kudzu.us>