net: lapbether: replace comparison to NULL with "lapbeth_get_x25_dev"

According to the chackpatch.pl, comparison to NULL could
be written "lapbeth_get_x25_dev".
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wan/lapbether.c

@@ -451,7 +451,7 @@ static int lapbeth_device_event(struct notifier_block *this,
 	switch (event) {
 	case NETDEV_UP:
 		/* New ethernet device -> new LAPB interface	 */
-		if (lapbeth_get_x25_dev(dev) == NULL)
+		if (!lapbeth_get_x25_dev(dev))
 			lapbeth_new_device(dev);
 		break;
 	case NETDEV_GOING_DOWN: