[AX.25]: Fix packet socket crash

Since changeset 98a82feb AX.25 is passing
received IP and ARP packets to the stack through netif_rx() but we don't
set the skb->mac.raw to right value which may result in a crash with
applications that use a packet socket.

Signed-off-by: Ralf Baechle DL5RB <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>