PCI: Make link retraining use RMW accessors for changing LNKCTL

Don't assume that the device is fully under the control of PCI core.  Use
RMW capability accessors in link retraining which do proper locking to
avoid losing concurrent updates to the register values.

Suggested-by: Lukas Wunner <lukas@wunner.de>
Fixes: 4ec73791 ("PCI: Work around Pericom PCIe-to-PCI bridge Retrain Link erratum")
Fixes: 7d715a6c ("PCI: add PCI Express ASPM support")
Link: https://lore.kernel.org/r/20230717120503.15276-3-ilpo.jarvinen@linux.intel.com


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: "Rafael J. Wysocki" <rafael@kernel.org>