AP_ADC_AnalogSource: fixed reference scaling for APM1 Ch6 ADC

it is not referenced to Vcc

libraries/AP_ADC_AnalogSource/AP_ADC_AnalogSource.cpp

@@ -22,10 +22,10 @@ float AP_ADC_AnalogSource::read_average() {
  */
 float AP_ADC_AnalogSource::voltage_average()
 {
-    float vcc_mV = hal.analogin->channel(ANALOG_INPUT_BOARD_VCC)->read_average();
     float fullscale = _adc->Ch(_ch);
-    // note that the Ch6 ADC on APM1 has a 3.3V range
-    return fullscale * vcc_mV * (3.3/5.0) * 2.44140625e-7f; // 1.0/(4096*1000)
+    // note that the Ch6 ADC on APM1 has a 3.3V range, and is against
+    // an internal reference, not the 5V power supply
+    return fullscale * 3.3 * 2.44140625e-4f;
 }