unused variable warning in LoRaMac.h for release build

In release builds where MBED_ASSERT macro is syphoned off, we were hitting
an unused variable warning. To mitigate that we could direct the variable
used in MBED_ASSERT to void.

features/lorawan/lorastack/mac/LoRaMac.h

@@ -475,11 +475,13 @@ public:
     {
         osStatus status = _mutex.lock();
         MBED_ASSERT(status == osOK);
+        (void) status;
     }
     void unlock(void)
     {
         osStatus status = _mutex.unlock();
         MBED_ASSERT(status == osOK);
+        (void) status;
     }
 #else
     void lock(void) { }