PNP: make pnp_bus_type const

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

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>