ag71xx: Move timestamp struct member outside of struct.

With this change, the timestamp variable is only used in ag71xx_check_dma_stuck. Small tx speedup.

Based on a Qualcomm commit. ag->timestamp = jiffies was not replaced with netif_trans_update(dev) because of this quote:

It should be noted that after this series several instances
of netif_trans_update() are useless (if they occur in
.ndo_start_xmit and driver doesn't set LLTX flag -- stack already
did an update).

From: http://lists.openwall.net/netdev/2016/05/03/87

Signed-off-by: Rosen Penev <rosenp@gmail.com>

target/linux/ar71xx/files/drivers/net/ethernet/atheros/ag71xx/ag71xx.h

@@ -169,8 +169,6 @@ struct ag71xx {
 	struct napi_struct	napi;
 	u32			msg_enable;
 
-	unsigned long	timestamp;
-
 	/*
 	 * From this point onwards we're not looking at per-packet fields.
 	 */

target/linux/ar71xx/files/drivers/net/ethernet/atheros/ag71xx/ag71xx_main.c

@@ -816,7 +816,6 @@ static netdev_tx_t ag71xx_hard_start_xmit(struct sk_buff *skb,
 	i = (ring->curr + n - 1) & ring_mask;
 	ring->buf[i].len = skb->len;
 	ring->buf[i].skb = skb;
-	ag->timestamp = jiffies;
 
 	netdev_sent_queue(dev, skb->len);
 
@@ -928,9 +927,11 @@ static void ag71xx_restart_work_func(struct work_struct *work)
 
 static bool ag71xx_check_dma_stuck(struct ag71xx *ag)
 {
+	unsigned long timestamp;
 	u32 rx_sm, tx_sm, rx_fd;
 
-	if (likely(time_before(jiffies, ag->timestamp + HZ/10)))
+	timestamp = netdev_get_tx_queue(ag->dev, 0)->trans_start;
+	if (likely(time_before(jiffies, timestamp + HZ/10)))
 		return false;
 
 	if (!netif_carrier_ok(ag->dev))