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/l3gd20/l3gd20.cpp

@@ -411,7 +411,7 @@ L3GD20::L3GD20(int bus, const char* path, spi_dev_e device, enum Rotation rotati
 	_gyro_scale{},
 	_gyro_range_scale(0.0f),
 	_gyro_range_rad_s(0.0f),
-	_gyro_topic(0),
+	_gyro_topic(nullptr),
 	_orb_class_instance(-1),
 	_class_instance(-1),
 	_current_rate(0),
@@ -490,7 +490,7 @@ L3GD20::init()
 	_gyro_topic = orb_advertise_multi(ORB_ID(sensor_gyro), &grp,
 		&_orb_class_instance, (is_external()) ? ORB_PRIO_VERY_HIGH : ORB_PRIO_DEFAULT);
 
-	if (_gyro_topic == 0) {
+	if (_gyro_topic == nullptr) {
 		debug("failed to create sensor_gyro publication");
 	}