ekf2: in bias estimator, use psd instead of var for prediction

PSD is independent from the sampling time while variance isn't

src/modules/ekf2/EKF/bias_estimator.cpp

@@ -43,7 +43,7 @@ void BiasEstimator::predict(const float dt)
 {
 	// State is constant
 	// Predict state covariance only
-	float delta_state_var = _process_var * dt * dt;
+	float delta_state_var = _process_psd * dt;
 
 	if (isOffsetDetected()) {
 		// A bias in the state has been detected by the innovation sequence check