can: c_can: Fix tx_bytes accounting

The current SocketCAN implementation for the Bosch c_can cell doesn't
account the TX bytes correctly, because it calls
c_can_inval_msg_object() (which clears the msg ctrl register) before
reading the DLC value:

for (/* nix */; (priv->tx_next - priv->tx_echo) > 0; priv->tx_echo++) {
        msg_obj_no = get_tx_echo_msg_obj(priv);
        c_can_inval_msg_object(dev, 0, msg_obj_no);
        val = c_can_read_reg32(priv, &priv->regs->txrqst1);
        if (!(val & (1 << msg_obj_no))) {
                can_get_echo_skb(dev,
                                msg_obj_no - C_CAN_MSG_OBJ_TX_FIRST);
                stats->tx_bytes += priv->read_reg(priv,
                                &priv->regs->ifregs[0].msg_cntrl)
                                & IF_MCONT_DLC_MASK;
                stats->tx_packets++;
        }
}

So, we will always read 0 for the DLC value and "ifconfig" will report
*0* TX Bytes.

The fix is quite easy: Just move c_can_inval_msg_object() to the end of
the if() statement. So:
      * We only call c_can_inval_msg_object() if the message was
        actually transmitted
      * We read out the DLC value _before_ clearing the msg ctrl
        register

Signed-off-by: Jan Altenberg <jan@linutronix.de>
Acked-by: Kurt Van Dijck <kurt.van.dijck@eia.be>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/can/c_can/c_can.c

@@ -699,7 +699,6 @@ static void c_can_do_tx(struct net_device *dev)
 
 	for (/* nix */; (priv->tx_next - priv->tx_echo) > 0; priv->tx_echo++) {
 		msg_obj_no = get_tx_echo_msg_obj(priv);
-		c_can_inval_msg_object(dev, 0, msg_obj_no);
 		val = c_can_read_reg32(priv, &priv->regs->txrqst1);
 		if (!(val & (1 << msg_obj_no))) {
 			can_get_echo_skb(dev,
@@ -708,6 +707,7 @@ static void c_can_do_tx(struct net_device *dev)
 					&priv->regs->ifregs[0].msg_cntrl)
 					& IF_MCONT_DLC_MASK;
 			stats->tx_packets++;
+			c_can_inval_msg_object(dev, 0, msg_obj_no);
 		}
 	}