net: ethernet: mlx4: Avoid assigning a value to ring_cons but not used it anymore in mlx4_en_xmit()

We found a set but not used variable 'ring_cons' in mlx4_en_xmit(), it will
cause a warning when build the kernel. And after checking the commit record
of this function, we found that it was introduced by a previous patch.

So, We delete this redundant assignment code.

Fixes: 488a9b48 ("net/mlx4_en: Wake TX queues only when there's enough room")
Signed-off-by: Luo Jiaxing <luojiaxing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_tx.c

@@ -1075,7 +1075,6 @@ netdev_tx_t mlx4_en_xmit(struct sk_buff *skb, struct net_device *dev)
 		 */
 		smp_rmb();
 
-		ring_cons = READ_ONCE(ring->cons);
 		if (unlikely(!mlx4_en_is_tx_ring_full(ring))) {
 			netif_tx_wake_queue(ring->tx_queue);
 			ring->wake_queue++;