atm: Introduce vcc_process_recv_queue

This function moves the implementation found in the clip and br2684
modules to common code, correctly unlinks the skb from the queue
before pushing it and makes pppoatm use it.

Signed-off-by: Jorge Boncompte [DTI2] <jorge@dti2.net>
Signed-off-by: David S. Miller <davem@davemloft.net>