tests/gnrc_sixlowpan: Use NETOPT_MAX_DPU_SIZE

Updated the test to use NETOPT_MAX_DPU_SIZE instead of the deprecated
NETOPT_MAX_PACKET_SIZE

tests/gnrc_sixlowpan/main.c

@@ -91,7 +91,7 @@ static void _init_interface(void)
     netdev_test_setup(&_ieee802154_dev, NULL);
     netdev_test_set_get_cb(&_ieee802154_dev, NETOPT_DEVICE_TYPE,
                            _get_netdev_device_type);
-    netdev_test_set_get_cb(&_ieee802154_dev, NETOPT_MAX_PACKET_SIZE,
+    netdev_test_set_get_cb(&_ieee802154_dev, NETOPT_MAX_PDU_SIZE,
                            _get_netdev_max_packet_size);
     netdev_test_set_get_cb(&_ieee802154_dev, NETOPT_SRC_LEN,
                            _get_netdev_src_len);