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/mb12xx/mb12xx.cpp

@@ -623,9 +623,9 @@ MB12XX::start()
 		true,
 		SUBSYSTEM_TYPE_RANGEFINDER
 	};
-	static orb_advert_t pub = -1;
+	static orb_advert_t pub = nullptr;
 
-	if (pub > 0) {
+	if (pub != nullptr) {
 		orb_publish(ORB_ID(subsystem_info), pub, &info);