net: fec: update dirty_tx even if no skb

If dirty_tx isn't updated, then dma_unmap_single
can be called twice.

This fixes a
[   58.420980] ------------[ cut here ]------------
[   58.425667] WARNING: CPU: 0 PID: 377 at /home/schurig/d/mkarm/linux-4.5/lib/dma-debug.c:1096 check_unmap+0x9d0/0xab8()
[   58.436405] fec 2188000.ethernet: DMA-API: device driver tries to free DMA memory it has not allocated [device address=0x0000000000000000] [size=66 bytes]

encountered by Holger

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Tested-by: <holgerschurig@gmail.com>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/fec_main.c

@@ -1197,10 +1197,8 @@ fec_enet_tx_queue(struct net_device *ndev, u16 queue_id)
 					 fec16_to_cpu(bdp->cbd_datlen),
 					 DMA_TO_DEVICE);
 		bdp->cbd_bufaddr = cpu_to_fec32(0);
-		if (!skb) {
-			bdp = fec_enet_get_nextdesc(bdp, &txq->bd);
-			continue;
-		}
+		if (!skb)
+			goto skb_done;
 
 		/* Check for errors. */
 		if (status & (BD_ENET_TX_HB | BD_ENET_TX_LC |
@@ -1239,7 +1237,7 @@ fec_enet_tx_queue(struct net_device *ndev, u16 queue_id)
 
 		/* Free the sk buffer associated with this last transmit */
 		dev_kfree_skb_any(skb);
-
+skb_done:
 		/* Make sure the update to bdp and tx_skbuff are performed
 		 * before dirty_tx
 		 */