PCI/ASPM: Use distinct local vars in pcie_retrain_link()

Use separate local variables to hold the respective values retrieved from
the Link Control Register and the Link Status Register.  Improves
readability and it makes it possible for the compiler to detect actual
uninitialised use should this code change in the future.

[bhelgaas: reorder to clean up before exposing to PCI core]
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2306110252260.64925@angie.orcam.me.uk


Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>