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/ets_airspeed/ets_airspeed.cpp

@@ -183,7 +183,7 @@ ETSAirspeed::collect()
 	report.temperature = -1000.0f;
 	report.max_differential_pressure_pa = _max_differential_pressure_pa;
 
-	if (_airspeed_pub > 0 && !(_pub_blocked)) {
+	if (_airspeed_pub != nullptr && !(_pub_blocked)) {
 		/* publish it */
 		orb_publish(ORB_ID(differential_pressure), _airspeed_pub, &report);
 	}