PCI/ASPM: Return 0 or -ETIMEDOUT from pcie_retrain_link()

"pcie_retrain_link" is not a question with a true/false answer, so "bool"
isn't quite the right return type.  Return 0 for success or -ETIMEDOUT if
the retrain failed.  No functional change intended.

[bhelgaas: based on Ilpo's patch below]
Link: https://lore.kernel.org/r/20230502083923.34562-1-ilpo.jarvinen@linux.intel.com


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>