net/mlx5: fix build without soft counters

total_length is only visible when SOFT_COUNTERS are enabled

Fixes: 3f13f8c23a ("net/mlx5: support hardware TSO")
Cc: stable@dpdk.org

Signed-off-by: Thierry Herbelot <thierry.herbelot@6wind.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>

drivers/net/mlx5/mlx5_rxtx.c

@@ -524,7 +524,9 @@ mlx5_tx_burst(void *dpdk_txq, struct rte_mbuf **pkts, uint16_t pkts_n)
 						     0,
 					};
 					ds = 1;
+#ifdef MLX5_PMD_SOFT_COUNTERS
 					total_length = 0;
+#endif
 					k++;
 					goto next_wqe;
 				}