driver core: device.h: make struct bus_type a const *

Now that all users who accessed the bus_type structure in struct device
are properly using it as a const *, mark it as such so that no one can
modify it going forward anymore.

Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230313182918.1312597-35-gregkh@linuxfoundation.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>