PCI/ASPM: Don't retrain link if ASPM not possible

Some (defective) PCIe devices are not able to reliably do link retraining.

Check to see if ASPM is possible between link partners before configuring
common clocking, and doing the resulting link retraining.  If ASPM is not
possible, there is no reason to risk losing access to a device due to an
unnecessary link retraining.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>