PCI: Make pci_bus_type constant

PCI: Make pci_bus_type constant

Since commit d492cc2573a0 ("driver core: device.h: make struct bus_type a
const *"), the driver core can properly handle constant struct bus_type,
move the pci_bus_type variable to be a constant structure as well, placing
it into read-only memory which can not be modified at runtime.

Link: https://lore.kernel.org/r/20240823074202.139265-1-kunwu.chan@linux.dev
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>