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/airspeed/airspeed.cpp

@@ -86,8 +86,8 @@ Airspeed::Airspeed(int bus, int address, unsigned conversion_interval, const cha
 	_measure_ticks(0),
 	_collect_phase(false),
 	_diff_pres_offset(0.0f),
-	_airspeed_pub(0),
-	_subsys_pub(0),
+	_airspeed_pub(nullptr),
+	_subsys_pub(nullptr),
 	_class_instance(-1),
 	_conversion_interval(conversion_interval),
 	_sample_perf(perf_alloc(PC_ELAPSED, "airspeed_read")),
@@ -146,7 +146,7 @@ Airspeed::init()
 		/* measurement will have generated a report, publish */
 		_airspeed_pub = orb_advertise(ORB_ID(differential_pressure), &arp);
 
-		if (_airspeed_pub == 0)
+		if (_airspeed_pub == nullptr)
 			warnx("uORB started?");
 	}
 
@@ -367,7 +367,7 @@ Airspeed::update_status()
 			SUBSYSTEM_TYPE_DIFFPRESSURE
 		};
 
-		if (_subsys_pub == 0) {
+		if (_subsys_pub == nullptr) {
 			orb_publish(ORB_ID(subsystem_info), _subsys_pub, &info);
 		} else {
 			_subsys_pub = orb_advertise(ORB_ID(subsystem_info), &info);