atm: br2684: Fix oops due to skb->dev being NULL

This oops have been already fixed with commit

    27141666

    atm: [br2684] Fix oops due to skb->dev being NULL

    It happens that if a packet arrives in a VC between the call to open it on
    the hardware and the call to change the backend to br2684, br2684_regvcc
    processes the packet and oopses dereferencing skb->dev because it is
    NULL before the call to br2684_push().

but have been introduced again with commit

    b6211ae7

    atm: Use SKB queue and list helpers instead of doing it by-hand.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>