PCI: Don't clear d3cold_allowed for PCIe ports

The PCI core skips bridges and ports when the system is suspended.  The PCI
core checks return value of pci_has_subordinate() in pci_pm_suspend_noirq()
to skip all devices where it is non-zero (which means PCI bridges and PCIe
ports).

Since PCIe ports are never suspended in the first place, there is no need
to set d3cold_allowed for them.

Tested-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>