[ATM] br2684: Use seq_list_xxx helpers

The .show callback receives the list_head pointer now, not the struct
br2684_dev one.

Signed-off-by: Pavel Emelianov <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>