PCI: Constify pcibus_class

Constify pcibus_class. All users take a const struct class * argument.

Link: https://lore.kernel.org/r/5e01f46f-266f-4fb3-be8a-8cb9e566cd75@gmail.com


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