Fix issue related to downlink sequence counter rollover in LoRaWAN.

For the test, the downlink sequence counter of the downlink messages has been incremented by 1600 (< MAX_FCNT_GAP) several times. For example, until 64005 and following situation was shown:

Downlink sequence counter increased to 64005 (by 1600 steps)
The radio module accepts this, as expected.
After that, downlink sequence counter increased to 65536 (rollover for 32 bits sequence counter)
The radio module rejects this frame
With the change everything works fine.

Author: itziardelatorre

features/lorawan/lorastack/mac/LoRaMac.cpp

@@ -319,10 +319,13 @@ bool LoRaMac::message_integrity_check(const uint8_t *const payload,
     sequence_counter_prev = (uint16_t) * downlink_counter;
     sequence_counter_diff = sequence_counter - sequence_counter_prev;
     *downlink_counter += sequence_counter_diff;
-    if (sequence_counter < sequence_counter_prev) {
-        *downlink_counter += 0x10000;
+    
+    if (sequence_counter_diff >= _lora_phy->get_maximum_frame_counter_gap()) {
+        _mcps_indication.status = LORAMAC_EVENT_INFO_STATUS_DOWNLINK_TOO_MANY_FRAMES_LOST;
+        _mcps_indication.dl_frame_counter = *downlink_counter;
+        return false;
     }
-
+    
     // sizeof nws_skey must be the same as _params.keys.nwk_skey,
     _lora_crypto.compute_mic(payload, size - LORAMAC_MFR_LEN,
                              nwk_skey,
@@ -332,13 +335,7 @@ bool LoRaMac::message_integrity_check(const uint8_t *const payload,
     if (mic_rx != mic) {
         _mcps_indication.status = LORAMAC_EVENT_INFO_STATUS_MIC_FAIL;
         return false;
-    }
-
-    if (sequence_counter_diff >= _lora_phy->get_maximum_frame_counter_gap()) {
-        _mcps_indication.status = LORAMAC_EVENT_INFO_STATUS_DOWNLINK_TOO_MANY_FRAMES_LOST;
-        _mcps_indication.dl_frame_counter = *downlink_counter;
-        return false;
-    }
+    }   
 
     return true;
 }