atm: br2684: Make headroom and hard_header_len depend on the payload type

Routed payload requires less headroom than bridged payload.
So do not reallocate headroom if not needed.
Also, add worst case AAL5 overhead to netdev->hard_header_len.

Signed-off-by: Pascal Hambourg <pascal@plouf.fr.eu.org>
Signed-off-by: chas williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Signed-off-by: David S. Miller <davem@davemloft.net>