mag_cal: fix mag bias estimate to mag cal

- since last_us is set to 0 every time the bias is not observable, the
  total time was also reset -> needed 30 consecutive seconds in mag 3D
  to be declared "stable"
- after landing, the mag_aligned_in_flight flag is reset. Using this for
  bias validity makes it invalid before we have a chance to save it to
  the calibration.

src/lib/sensor_calibration/Magnetometer.cpp

@@ -110,7 +110,7 @@ void Magnetometer::SensorCorrectionsUpdate(bool force)
 
 bool Magnetometer::set_offset(const Vector3f &offset)
 {
-	if (Vector3f(_offset - offset).longerThan(0.01f)) {
+	if (Vector3f(_offset - offset).longerThan(0.005f)) {
 		if (offset.isAllFinite()) {
 			_offset = offset;
 			_calibration_count++;