can: c_can: add a comment about IF_RX interface's use

After reading the commit 640916db ("can: c_can: Make it SMP safe")
it may sound strange to see the IF_RX interface used by the
can_inval_tx_object function. A comment was added to avoid any
misunderstanding.

Link: https://lore.kernel.org/r/20210302215435.18286-4-dariobin@libero.itSigned-off-by: Dario Binacchi <dariobin@libero.it>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/c_can/c_can.c

@@ -710,6 +710,11 @@ static void c_can_do_tx(struct net_device *dev)
 		idx--;
 		pend &= ~(1 << idx);
 		obj = idx + C_CAN_MSG_OBJ_TX_FIRST;
+
+		/* We use IF_RX interface instead of IF_TX because we
+		 * are called from c_can_poll(), which runs inside
+		 * NAPI. We are not trasmitting.
+		 */
 		c_can_inval_tx_object(dev, IF_RX, obj);
 		can_get_echo_skb(dev, idx, NULL);
 		bytes += priv->dlc[idx];