PCI/ASPM: Factor out pcie_wait_for_retrain()

[ Upstream commit 9c7f136433d26592cb4d9cd00b4e15c33d9797c6 ]

Factor pcie_wait_for_retrain() out from pcie_retrain_link().  No functional
change intended.

[bhelgaas: split out from
https: //lore.kernel.org/r/20230502083923.34562-1-ilpo.jarvinen@linux.intel.com]
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Stable-dep-of: e7e39756363a ("PCI/ASPM: Avoid link retraining race")
Signed-off-by: Sasha Levin <sashal@kernel.org>