AP_Proximity: increase MAV driver timeout to 500ms

this comes after testing with ROS/mavros and discovering the update rate is only 4hz

libraries/AP_Proximity/AP_Proximity_MAV.cpp

@@ -21,6 +21,8 @@
 
 extern const AP_HAL::HAL& hal;
 
+#define PROXIMITY_MAV_TIMEOUT_MS    500 // distance messages must arrive within this many milliseconds
+
 /* 
    The constructor also initialises the proximity sensor. Note that this
    constructor is not called until detect() returns true, so we

libraries/AP_Proximity/AP_Proximity_MAV.h

@@ -3,8 +3,6 @@
 #include "AP_Proximity.h"
 #include "AP_Proximity_Backend.h"
 
-#define PROXIMITY_MAV_TIMEOUT_MS    200 // requests timeout after 0.2 seconds
-
 class AP_Proximity_MAV : public AP_Proximity_Backend
 {