PCI/ASPM: Use FIELD_MAX() instead of literals

Convert 0x3ff literals in encode_l12_threshold() to
FIELD_MAX(PCI_L1SS_CTL1_LTR_L12_TH_VALUE) that explains the purpose of
the literal.

Link: https://lore.kernel.org/r/20230915155752.84640-6-ilpo.jarvinen@linux.intel.com


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>