PCI/ASPM: Configure L1 substate settings

Configure the L1 substate settings on the upstream and downstream devices,
while taking care of the rules dictated by the PCIe spec.

[bhelgaas: drop "inline"]
Signed-off-by: Rajat Jain <rajatja@google.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>