Keep the Join counter at 0 when the first fails

When we fail to send the very first OTAA Join, the Join retransmission
logic must not set joins_left to a non-zero value. Otherwise, all future
joins would fail since the variable would never be reset to zero.

Closes #92

src/lrw.c

@@ -945,8 +945,9 @@ int lrw_join(uint8_t datarate, uint8_t tries)
 #ifdef RESTORE_CHMASK_AFTER_JOIN
         save_chmask();
 #endif
-        joins_left = tries;
-        return send_join();
+        LoRaMacStatus_t rc = send_join();
+        if (rc == LORAMAC_STATUS_OK) joins_left = tries;
+        return rc;
     }
 }