orb_advert_t changed to void * and checks changed to nullptr

The existing orb_advert_t use thoughout the code sometimes tries
to treat it as a file descriptor and there are checks for < 0
and ::close calls on orb_advert_t types which is an invalid use
of an object pointer, which is what orb_advert_t really is.

Initially I had changed the -1 initializations to 0 but it was
suggested that this should be nullptr. That was a good recommendation
but the definition of orb_advert_t had to change to void * because
you cannot initialize a uintptr_t as nullptr.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/drivers/trone/trone.cpp

@@ -301,7 +301,7 @@ TRONE::init()
 		_distance_sensor_topic = orb_advertise_multi(ORB_ID(distance_sensor), &ds_report,
 							     &_orb_class_instance, ORB_PRIO_LOW);
 
-		if (_distance_sensor_topic == 0) {
+		if (_distance_sensor_topic == nullptr) {
 			log("failed to create distance_sensor object. Did you start uOrb?");
 		}
 	}