PCI/PM: Allow PCI devices to be put into D3cold during system suspend

Commit 448bd857 ("PCI/PM: add PCIe runtime D3cold support") added a
check to prevent PCI devices from being put into D3cold during system
suspend without giving any particular reason.

Also the check isn't really necessary, because acpi_pci_set_power_state()
maps PCI_D3hot to ACPI_STATE_D3_COLD anyway.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>