EKF: Rework use of fuseDeclination()

Simplify calling so that it is only called in two ways:
1) Immediately before 3-axis mag fusion if not called since the last earth field covariance reset so that the earth field declination information can be formed.
2) Immediately after 3-axis mag fusion otherwise.

EKF/covariance.cpp

@@ -847,6 +847,7 @@ void Ekf::fixCovarianceErrors()
 	if (!_control_status.flags.mag_3D) {
 		zeroRows(P, 16, 21);
 		zeroCols(P, 16, 21);
+		_mag_decl_cov_reset = false;
 
 	} else {
 		// constrain variances
@@ -887,15 +888,13 @@ void Ekf::resetMagCovariance()
 	// set the magnetic field covariance terms to zero
 	zeroRows(P, 16, 21);
 	zeroCols(P, 16, 21);
+	_mag_decl_cov_reset = false;
 
 	// set the field state variance to the observation variance
 	for (uint8_t rc_index = 16; rc_index <= 21; rc_index ++) {
 		P[rc_index][rc_index] = sq(_params.mag_noise);
 	}
 
-	// Fuse the declination angle to prevent rapid rotation of earth field vector estimates
-	fuseDeclination(0.02f);
-
 	// save covariance data for re-use when auto-switching between heading and 3-axis fusion
 	save_mag_cov_data();
 }