gianfar: fix the last transmit buffer descriptor

When the transmit hardware timestamping is enabled, an additional
TxBD would be added and would be set as the last TxBD with TXBD_LAST
and TXBD_INTERRUPT. However this has been broken by a patch recently.
This made the software couldn't get transmit hardware timestamps and
resulted in call trace. So, this patch is to fix this issue.

Fixes: 48963b44 ("gianfar: Remove redundant ops for do_tstamp
       from xmit()")
Signed-off-by: NYangbo Lu <yangbo.lu@nxp.com>
Reviewed-by: NClaudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/gianfar.c

@@ -2440,7 +2440,8 @@ static int gfar_start_xmit(struct sk_buff *skb, struct net_device *dev)
 						 tx_queue->tx_ring_size);
 
 	if (likely(!nr_frags)) {
-		lstatus |= BD_LFLAG(TXBD_LAST | TXBD_INTERRUPT);
+		if (likely(!do_tstamp))
+			lstatus |= BD_LFLAG(TXBD_LAST | TXBD_INTERRUPT);
 	} else {
 		u32 lstatus_start = lstatus;