PCI/ASPM: Save parent L1SS config in pci_save_aspm_l1ss_state()

After 17423360 ("PCI/ASPM: Save L1 PM Substates Capability for
suspend/resume"), pci_save_aspm_l1ss_state(dev) saves the L1SS state for
"dev", and pci_restore_aspm_l1ss_state(dev) restores the state for both
"dev" and its parent.

The problem is that unless pci_save_state() has been used in some other
path and has already saved the parent L1SS state, we will restore junk to
the parent, which means the L1 Substates likely won't work correctly.

Save the L1SS config for both the device and its parent in
pci_save_aspm_l1ss_state().  When restoring, we need both because L1SS must
be enabled at the parent (the Downstream Port) before being enabled at the
child (the Upstream Port).

Link: https://lore.kernel.org/r/20241115072200.37509-3-jhp@endlessos.org
Fixes: 17423360 ("PCI/ASPM: Save L1 PM Substates Capability for suspend/resume")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218394


Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Jian-Hong Pan <jhp@endlessos.org>
[bhelgaas: parallel save/restore structure, simplify commit log, patch at
https://lore.kernel.org/r/20241212230340.GA3267194@bhelgaas

]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Jian-Hong Pan <jhp@endlessos.org> # Asus B1400CEAE