can: dev: fix sparse warning for can_restart

Make can_restart static to fix the following sparse warning:
drivers/net/can/dev.c:371:6: warning: symbol 'can_restart' was not
declared. Should it be static?
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/dev.c

@@ -401,7 +401,7 @@ EXPORT_SYMBOL_GPL(can_free_echo_skb);
 /*
  * CAN device restart for bus-off recovery
  */
-void can_restart(unsigned long data)
+static void can_restart(unsigned long data)
 {
 	struct net_device *dev = (struct net_device *)data;
 	struct can_priv *priv = netdev_priv(dev);