mkiss: Fix error handling in mkiss_open()

If register_netdev() fails we are not propagating the error and
we return success because ax_open() succeeded previously.

Fix this by checking the return value of ax_open() and
register_netdev() and propagate the error in case of failure.
Reported-by: NRUC_Soft_Sec <zy900702@163.com>
Signed-off-by: NFabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/hamradio/mkiss.c

@@ -728,11 +728,12 @@ static int mkiss_open(struct tty_struct *tty)
 	dev->type = ARPHRD_AX25;
 
 	/* Perform the low-level AX25 initialization. */
-	if ((err = ax_open(ax->dev))) {
+	err = ax_open(ax->dev);
+	if (err)
 		goto out_free_netdev;
-	}
 
-	if (register_netdev(dev))
+	err = register_netdev(dev);
+	if (err)
 		goto out_free_buffers;
 
 	/* after register_netdev() - because else printk smashes the kernel */