can: dev: can_restart(): convert NULL pointer check

This patch converts the NULL pointer check in can_restart() form "skb ==
NULL" to "!skb".
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/dev.c

@@ -552,7 +552,7 @@ static void can_restart(struct net_device *dev)
 
 	/* send restart message upstream */
 	skb = alloc_can_err_skb(dev, &cf);
-	if (skb == NULL) {
+	if (!skb) {
 		err = -ENOMEM;
 		goto restart;
 	}