LoRa: Fix build warning about _ongoing_tx_msg

_ongoing_tx_msg was incorrectly initialized before _mlme_confirmation.

Fixes following build warning:
[Warning] LoRaMac.h@691,26: 'LoRaMac::_ongoing_tx_msg' will be initialized after [-Wreorder]
[Warning] LoRaMac.h@689,28:   'loramac_mlme_confirm_t LoRaMac::_mlme_confirmation' [-Wreorder]
[Warning] LoRaMac.cpp@68,1:   when initialized here [-Wreorder]

Author: Kimmo Vaisanen

features/lorawan/lorastack/mac/LoRaMac.cpp

@@ -76,8 +76,8 @@ LoRaMac::LoRaMac()
       _mcps_indication(),
       _mcps_confirmation(),
       _mlme_indication(),
-      _ongoing_tx_msg(),
       _mlme_confirmation(),
+      _ongoing_tx_msg(),
       _is_nwk_joined(false),
       _can_cancel_tx(true),
       _continuous_rx2_window_open(false),