AP_Notify: don't use pinMode for pixracer LEDs

this is needed for LEDs that are driven by 5V, which means they need
to use OPENDRAIN.

libraries/AP_Notify/PixRacerLED.cpp

@@ -36,9 +36,13 @@ PixRacerLED::PixRacerLED() :
 
 bool PixRacerLED::hw_init(void)
 {
+    // when HAL_GPIO_LED_ON is 0 then we must not use pinMode()
+    // as it could remove the OPENDRAIN attribute on the pin
+#if HAL_GPIO_LED_ON != 0
     hal.gpio->pinMode(HAL_GPIO_A_LED_PIN, HAL_GPIO_OUTPUT);
     hal.gpio->pinMode(HAL_GPIO_B_LED_PIN, HAL_GPIO_OUTPUT);
     hal.gpio->pinMode(HAL_GPIO_C_LED_PIN, HAL_GPIO_OUTPUT);
+#endif
     hal.gpio->write(HAL_GPIO_A_LED_PIN, HAL_GPIO_LED_OFF);
     hal.gpio->write(HAL_GPIO_B_LED_PIN, HAL_GPIO_LED_OFF);
     hal.gpio->write(HAL_GPIO_C_LED_PIN, HAL_GPIO_LED_OFF);