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

[ Upstream commit a9a18e8f770c9b0703dab93580d0b02e199a4c79 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>