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().

Signed-off-by: Jorge Boncompte [DTI2] <jorge@dti2.net>
Signed-off-by: Chas Williams <chas@cmf.nrl.navy.mil>