PCI: Revert to the original speed after PCIe failed link retraining

PCI: Revert to the original speed after PCIe failed link retraining

When `pcie_failed_link_retrain' has failed to retrain the link by hand
it leaves the link speed restricted to 2.5GT/s, which will then affect
any device that has been plugged in later on, which may not suffer from
the problem that caused the speed restriction to have been attempted.
Consequently such a downstream device will suffer from an unnecessary
communication throughput limitation and therefore performance loss.

Remove the speed restriction then and revert the Link Control 2 register
to its original state if link retraining with the speed restriction in
place has failed.  Retrain the link again afterwards so as to remove any
residual state, waiting on LT rather than DLLLA to avoid an excessive
delay and ignoring the result as this training is supposed to fail
anyway.

Fixes: a89c82249c37 ("PCI: Work around PCIe link training failures")
Link: https://lore.kernel.org/linux-pci/alpine.DEB.2.21.2408251412590.30766@angie.orcam.me.uk
Reported-by: Matthew W Carlis <mattc@purestorage.com>
Link: https://lore.kernel.org/r/20240806000659.30859-1-mattc@purestorage.com/
Link: https://lore.kernel.org/r/20240722193407.23255-1-mattc@purestorage.com/
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Cc: <stable@vger.kernel.org> # v6.5+