Let MAC choose address when mesh forwarding

We accidentally copied the source address form (16- or 64-bit) when
forwarding a mesh packet, so if the incoming packet had a 64-bit source,
we used a 64-bit source.

Clear the address type field before selecting address, so we use the
default behaviour of "use 16-bit if available".

This means we will use 16-bit in Thread - eliminates our involvement in
the case of one vendor rejecting mesh packets with 64-bit MAC source.

Change-Id: I77b925cd6efd43095c5fc8c00a63abda5f908ffa

Author: kjbracey

source/6LoWPAN/Mesh/mesh.c

@@ -243,6 +243,7 @@ buffer_t *mesh_up(buffer_t *buf)
     memcpy(buf->dst_sa.address + 2, route.address, route.addr_len);
 
     /* Set src MAC address from our interface */
+    buf->src_sa.addr_type = ADDR_NONE;
     if (!mac_helper_write_our_addr(cur, &buf->src_sa)) {
         goto drop;
     }