wan: dlci/sdla transmit return dehacking

This is a brute force removal of the wierd slave interface done for
DLCI -> SDLA transmit. Before it was using non-standard return values
and freeing skb in caller.  This changes it to using normal return
values, and freeing in the callee.  Luckly only one driver pair was
doing this. Not tested on real hardware, in fact I wonder if this
driver pair is even being used by any users.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>