net: arc_emac: reset txbd_curr and txbd_dirty pointers to zero

EMAC reset internal tx ring pointer to zero at statup.
txbd_curr and txbd_dirty can be different from zero.
That cause ethernet transfer hang (no packets transmitted).

In order to reproduce, run on device:
    ifconfig eth0 down
    ifconfig eth0 up
Signed-off-by: NAlexander Kochetkov <al.kochet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/arc/emac_main.c

@@ -447,6 +447,9 @@ static int arc_emac_open(struct net_device *ndev)
 		*last_rx_bd = (*last_rx_bd + 1) % RX_BD_NUM;
 	}
 
+	priv->txbd_curr = 0;
+	priv->txbd_dirty = 0;
+
 	/* Clean Tx BD's */
 	memset(priv->txbd, 0, TX_RING_SZ);