PCI: Annotate pci_cache_line_size variables as __ro_after_init

Annotate both variables as __ro_after_init, enforcing that they can't be
changed after the init phase.

Link: https://lore.kernel.org/r/52fd058d-6d72-48db-8e61-5fcddcd0aa51@gmail.com


Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>