hamradio: 6pack: fix error return code

Set the return variable to an error code as done elsewhere in the function.

A simplified version of the semantic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
(
if@p1 (\(ret < 0\|ret != 0\))
 { ... return ret; }
|
ret@p1 = 0
)
... when != ret = e1
    when != &ret
*if(...)
{
  ... when != ret = e2
      when forall
 return ret;
}

// </smpl>
Signed-off-by: NJulia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/hamradio/6pack.c

@@ -662,7 +662,8 @@ static int sixpack_open(struct tty_struct *tty)
 	tty->receive_room = 65536;
 
 	/* Now we're ready to register. */
-	if (register_netdev(dev))
+	err = register_netdev(dev);
+	if (err)
 		goto out_free;
 
 	tnc_init(sp);