EKF: Fix inconsistency in timeout for terrain validity

Terrain validity is determined solely by successful range finder fusion and terrain state initialisation.
A range finder that has been declared faulty requires continuous range finder data fusion requires  data to be continuous before the fault status _rng_hgt_faulty  can be cleared. This will enforce the requirement for continuous data before fusion can commence.

EKF/control.cpp

@@ -1144,6 +1144,11 @@ void Ekf::checkRangeDataValidity()
 		return;
 	}
 
+	// Don't allow faulty flag to clear unless range data is continuous
+	if (_rng_hgt_faulty && !_range_data_continuous) {
+		return;
+	}
+
 	// Don't run the checks after this unless we have retrieved new data from the buffer
 	if (!_range_data_ready) {
 		return;