AP_NavEKF: Relax timeout check applied to optical flow data

200 msec was too short and could lead to false positives. 5000 msec is the largest time we can go free inertial.

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -4418,7 +4418,7 @@ bool NavEKF::useRngFinder(void) const
 // return true if optical flow data is available
 bool NavEKF::optFlowDataPresent(void) const
 {
-    if (imuSampleTime_ms - flowMeaTime_ms < 200) {
+    if (imuSampleTime_ms - flowMeaTime_ms < 5000) {
         return true;
     } else {
         return false;