PCI/ASPM: Use 'parent' and 'child' for readability

Other users of link->pdev and link->downstream, e.g., pcie_aspm_cap_init(),
pcie_config_aspm_l1ss(), and pcie_config_aspm_link(), use "parent" and
"child" as local names.

Do the same in aspm_calc_l1ss_info() for readability.  No functional change
intended.

Link: https://lore.kernel.org/r/20201015193039.12585-4-helgaas@kernel.org


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