pcie_get_aspm_reg() mostly reads ASPM-related registers, but in some cases
it also updates the value read from PCI_L1SS_CAP based on LTR properties.

Move this update to the point where the value is used to make the code more
readable.

No functional change intended, although previously we could clear
PCI_L1SS_CAP_ASPM_L1_2 for both ends of the link, and now we'll only do it
for the downstream end of a link.  This shouldn't matter because we always
test that bit by ANDing l1ss_cap for the upstream and downstream ends.

Link: https://lore.kernel.org/r/20201015193039.12585-3-helgaas@kernel.orgSigned-off-by: Bjorn Helgaas <bhelgaas@google.com>