PCI/ASPM: Factor out L1 PM Substates configuration

Move L1 PM Substates configuration from pcie_aspm_cap_init() to a new
aspm_l1ss_init() function.  No functional change intended.

Link: https://lore.kernel.org/r/20221005025809.2247547-2-helgaas@kernel.org


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>