GCS_MAVLink: correct return type on get_stream_slowdown_ms

Only used in one place.  On slow links may have caused us to re-request
a waypoint too often.  "Too often" is still >1s intervals.

libraries/GCS_MAVLink/GCS.h

@@ -292,7 +292,7 @@ public:
     static const struct stream_entries all_stream_entries[];
 
     virtual uint64_t capabilities() const;
-    uint8_t get_stream_slowdown_ms() const { return stream_slowdown_ms; }
+    uint16_t get_stream_slowdown_ms() const { return stream_slowdown_ms; }
 
     MAV_RESULT set_message_interval(uint32_t msg_id, int32_t interval_us);