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: Julia Lawall <julia@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>