atsam: ADC pins need to be in input mode with pullups disabled

The ADC pins (on at least the sam3x8e) need to be configured in input
mode with the internal pullups disabled in order to get accurate ADC
readings.

Reported by @bryanboettcher.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/atsam/adc.c

@@ -49,6 +49,9 @@ gpio_adc_setup(uint8_t pin)
                        | ADC_MR_STARTUP_SUT768
                        | ADC_MR_TRANSFER(1));
     }
+
+    // Place pin in input floating mode
+    gpio_in_setup(pin, 0);
     return (struct gpio_adc){ .chan = 1 << chan };
 }