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/stm32/adc/adc.cpp

@@ -148,7 +148,7 @@ ADC::ADC(uint32_t channels) :
 	_sample_perf(perf_alloc(PC_ELAPSED, "adc_samples")),
 	_channel_count(0),
 	_samples(nullptr),
-	_to_system_power(0)
+	_to_system_power(nullptr)
 {
 	_debug_enabled = true;
 
@@ -329,7 +329,7 @@ ADC::update_system_power(void)
 	system_power.hipower_5V_OC = !stm32_gpioread(GPIO_VDD_5V_HIPOWER_OC);
 
 	/* lazily publish */
-	if (_to_system_power > 0) {
+	if (_to_system_power != nullptr) {
 		orb_publish(ORB_ID(system_power), _to_system_power, &system_power);
 	} else {
 		_to_system_power = orb_advertise(ORB_ID(system_power), &system_power);