drivers/atm/atmtcp.c: add missing atm_dev_put

The earlier call to atm_dev_lookup increases the reference count of dev,
so decrease it on the way out.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression x, E;
constant C;
@@

x = atm_dev_lookup(...);
... when != false x != NULL
    when != true x == NULL
    when != \(E = x\|x = E\)
    when != atm_dev_put(dev);
*return -C;
// </smpl>
Signed-off-by: NJulia Lawall <julia@diku.dk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/atm/atmtcp.c

@@ -392,7 +392,10 @@ static int atmtcp_attach(struct atm_vcc *vcc,int itf)
 			atm_dev_put(dev);
 			return -EMEDIUMTYPE;
 		}
-		if (PRIV(dev)->vcc) return -EBUSY;
+		if (PRIV(dev)->vcc) {
+			atm_dev_put(dev);
+			return -EBUSY;
+		}
 	}
 	else {
 		int error;