HAL_AVR: configure analog inputs as INPUT without pullup

not all pins were defaulting to this, which led to some strange
readings on some devices. First noticed by Andi on a LM335 temperature
sensor

libraries/AP_HAL_AVR/AnalogIn_ADC.cpp

@@ -15,10 +15,12 @@ ADCSource::ADCSource(uint8_t pin) :
     _sum_count(0),
     _sum(0),
     _last_average(0),
-    _pin(pin),
+    _pin(ANALOG_INPUT_NONE),
     _stop_pin(ANALOG_INPUT_NONE),
     _settle_time_ms(0)
-{}
+{
+    set_pin(pin);
+}
 
 float ADCSource::read_average() {
     if (_pin == ANALOG_INPUT_BOARD_VCC) {
@@ -70,7 +72,26 @@ float ADCSource::voltage_average_ratiometric(void)
 }
 
 void ADCSource::set_pin(uint8_t pin) {
-    _pin = pin;
+    if (pin != _pin) {
+        // ensure the pin is marked as an INPUT pin
+        if (pin != ANALOG_INPUT_NONE && pin != ANALOG_INPUT_BOARD_VCC) {
+            int8_t dpin = hal.gpio->analogPinToDigitalPin(pin);
+            if (dpin != -1) {
+                // enable as input without a pull-up. This gives the
+                // best results for our analog sensors
+                hal.gpio->pinMode(dpin, GPIO_INPUT);
+                hal.gpio->write(dpin, 0);
+            }
+        }
+        uint8_t sreg = SREG;
+        cli();
+        _sum = 0;
+        _sum_count = 0;
+        _last_average = 0;
+        _latest = 0;
+        _pin = pin;
+        SREG = sreg;
+    }
 }
 
 void ADCSource::set_stop_pin(uint8_t pin) {