packetbuf: Use packetbuf_holds_broadcast() all-over

core/net/mac/sicslowmac/sicslowmac.c

@@ -123,11 +123,7 @@ send_packet(mac_callback_t sent, void *ptr)
   params.fcf.src_addr_mode = FRAME802154_LONGADDRMODE;
   params.dest_pid = mac_dst_pan_id;
 
-  /*
-   *  If the output address is NULL in the Rime buf, then it is broadcast
-   *  on the 802.15.4 network.
-   */
-  if(linkaddr_cmp(packetbuf_addr(PACKETBUF_ADDR_RECEIVER), &linkaddr_null)) {
+  if(packetbuf_holds_broadcast()) {
     /* Broadcast requires short address mode. */
     params.fcf.dest_addr_mode = FRAME802154_SHORTADDRMODE;
     params.dest_addr[0] = 0xFF;