Fixed Route cost calculation

If route cost was larger than 0xffff it was calculated incorrectly and
wrapped

Author: Mika Tervonen

source/6LoWPAN/ws/ws_bootstrap.c

@@ -2645,6 +2645,10 @@ static uint16_t ws_bootstrap_routing_cost_calculate(protocol_interface_info_entr
         //Scale to 128 based ETX (local read retur 0x100 - 0xffff
         etx = etx >> 1;
     }
+    // Make the 0xffff as maximum value
+    if (ws_neighbor->routing_cost + etx > 0xffff) {
+        return 0xffff;
+    }
 
     return ws_neighbor->routing_cost + etx;
 }