PPP, ppp->last_xmit is only used in PPPoS, moved to PPPoS

Only PPPoS need to keep track of last transmitted packet for HDLC flag,
removed from PPPoE and PPPoL2TP and moved to PPPoS.

src/include/netif/ppp/ppp.h

@@ -347,8 +347,6 @@ struct ppp_pcb_s {
   unsigned int lcp_echo_timer_running  :1; /* set if a timer is running */
   unsigned int                         :2; /* 2 bits of padding to round out to 8 bits */
 
-  u32_t last_xmit;               /* Time of last transmission. */
-
   /* auth data */
 #if PPP_SERVER
   char peer_authname[MAXNAMELEN + 1]; /* The name by which the peer authenticated itself to us. */

src/include/netif/ppp/pppos.h

@@ -71,9 +71,10 @@ struct pppos_pcb_s {
 
   /* -- below are data that will be cleared between two sessions
    *
-   * out_accm must be the first member of cleared members, because it is
+   * last_xmit must be the first member of cleared members, because it is
    * used to know which part must not be cleared.
    */
+  u32_t last_xmit;                 /* Time of last transmission. */
   ext_accm out_accm;               /* Async-Ctl-Char-Map for output. */
 
   /* flags */