EKF: Don't start using mag if optical flow use is interrupted

If magnetometer use is being inhibited because an indoor condition has been detected (optical flow and poor or no GPS) and the optical flow fusion checks fail causing the EKF to stop navigating, we do not want the EKF to start using the magnetometer.

EKF/control.cpp

@@ -1396,10 +1396,10 @@ void Ekf::controlMagFusion()
 		// If optical flow is the only aiding source and GPS checks are failing, then assume that we are operating
 		// indoors and the magnetometer is unreliable. Becasue the optical flow sensor is body fixed, absolute yaw
 		// wrt North is not required for navigation and it is safer not to use the magnetometer.
-		if (_control_status.flags.opt_flow
+		if ((_control_status.flags.opt_flow || _mag_use_inhibit)
 				&& !_control_status.flags.gps
 				&& !_control_status.flags.ev_pos
-				&& ((_time_last_imu - _last_gps_pass_us) > 5E6)) {
+				&& (_time_last_imu - _last_gps_pass_us > (uint64_t)5e6)) {
 			_mag_use_inhibit = true;
 		} else {
 			_mag_use_inhibit = false;