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/bma180/bma180.cpp

@@ -238,7 +238,7 @@ BMA180::BMA180(int bus, spi_dev_e device) :
 	_reports(nullptr),
 	_accel_range_scale(0.0f),
 	_accel_range_m_s2(0.0f),
-	_accel_topic(0),
+	_accel_topic(nullptr),
 	_class_instance(-1),
 	_current_lowpass(0),
 	_current_range(0),
@@ -733,7 +733,7 @@ BMA180::measure()
 	poll_notify(POLLIN);
 
 	/* publish for subscribers */
-	if (_accel_topic != 0 && !(_pub_blocked))
+	if (_accel_topic != nullptr && !(_pub_blocked))
 		orb_publish(ORB_ID(sensor_accel), _accel_topic, &report);
 
 	/* stop the perf counter */