PCI/ASPM: Add pcie_aspm_get_link()

Factor out getting the link associated with a pci_dev and use this helper
where appropriate.  In addition this helper will be used in a subsequent
patch of this series.

Link: https://lore.kernel.org/r/19d33770-29de-a9af-4d85-f2b30269d383@gmail.com


Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>