nubus: Make nubus_bus_type static and constant

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

It's also never used outside of drivers/nubus/bus.c so make it static
and don't export it as no one is using it.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Finn Thain <fthain@linux-m68k.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/2023121940-enlarged-editor-c9a8@gregkh


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>