Fixed Join Request retransmission timing (Interop test)

On interop test 1.2.2.4, Join Request retransmission is expected to be 6
s + worst case air transmission. This delay is to accommodate for
JoinAccept through RX2.

The `call_in` in process_reception_timeout of RX2-window adds 500 ms
delay between RX2 symbol-interrupt-timeout and the next join request
retransmission. This is an isolated change and only affect the
retransmission of Join Request.

Adding this delay improves the chance of succeeding test
1.2.2.4 (subset of 1.2.2)

Author: zul00

connectivity/lorawan/source/LoRaWANStack.cpp

@@ -756,7 +756,10 @@ void LoRaWANStack::process_reception_timeout(bool is_timeout)
     _loramac.on_radio_rx_timeout(is_timeout);
 
     if (slot == RX_SLOT_WIN_2 && !_loramac.nwk_joined()) {
-        state_controller(DEVICE_STATE_JOINING);
+        const int ret = _queue->call_in(
+                            500, this, &LoRaWANStack::state_controller, DEVICE_STATE_JOINING);
+        MBED_ASSERT(ret != 0);
+        (void)ret;
         return;
     }