atm: idt77252: prevent use after free in dequeue_rx()

We can't dereference "skb" after calling vcc->push() because the skb
is released.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>