cxgb4: Use ACCES_ONCE macro to read queue's consumer index

Use helper macro ACCESS_ONCE() to load from the SGE status page
to prevent the compiler loading multiple times.

Based on original work by Mike Werner <werner@chelsio.com>

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/chelsio/cxgb4/sge.c

@@ -406,7 +406,7 @@ static void free_tx_desc(struct adapter *adap, struct sge_txq *q,
  */
 static inline int reclaimable(const struct sge_txq *q)
 {
-	int hw_cidx = ntohs(q->stat->cidx);
+	int hw_cidx = ntohs(ACCESS_ONCE(q->stat->cidx));
 	hw_cidx -= q->cidx;
 	return hw_cidx < 0 ? hw_cidx + q->size : hw_cidx;
 }
@@ -1320,7 +1320,7 @@ out_free:	dev_kfree_skb_any(skb);
  */
 static inline void reclaim_completed_tx_imm(struct sge_txq *q)
 {
-	int hw_cidx = ntohs(q->stat->cidx);
+	int hw_cidx = ntohs(ACCESS_ONCE(q->stat->cidx));
 	int reclaim = hw_cidx - q->cidx;
 
 	if (reclaim < 0)