atm: unset IFF_XMIT_DST_RELEASE in clip_setup()

clip_start_xmit() needs skb->dst so tell dev_hard_start_xmit()
to no release it.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>