PCI: endpoint: Make pci_epf_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the pci_epf_bus_type variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20240204-bus_cleanup-pci-v1-1-300267a1e99e@marliere.net


[mani: modified subject to reflect subsys prefix]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>