zorro: Make zorro_bus_type const

Since commit d492cc25 ("driver core: device.h: make struct
bus_type a const *"), the driver core can properly handle constant
struct bus_type, move the zorro_bus_type variable to be a constant
structure as well, placing it into read-only memory which can not be
modified at runtime.

Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20240213-bus_cleanup-zorro-v1-1-388ceed8e7bd@marliere.net


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