can: mscan: mark expected switch fall-through

As -Wimplicit-fallthrough is now enabled the mscan driver shows this warning,
which is fixed by this patch:

drivers/net/can/mscan/mscan.c: In function ‘mscan_start_xmit’:
drivers/net/can/mscan/mscan.c:211:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
  211 |   netif_stop_queue(dev);
      |   ^~~~~~~~~~~~~~~~~~~~~
drivers/net/can/mscan/mscan.c:212:2: note: here
  212 |  case 2:
      |  ^~~~

Link: https://lore.kernel.org/r/20200915223527.1417033-17-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/mscan/mscan.c

@@ -209,6 +209,7 @@ static netdev_tx_t mscan_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		 * since buffer with lower id have higher priority (hell..)
 		 */
 		netif_stop_queue(dev);
+		fallthrough;
 	case 2:
 		if (buf_id < priv->prev_buf_id) {
 			priv->cur_pri++;