PPP, MPPE, improve dropping of unencrypted received packet

Improve dropping of unencrypted received packet by taking into account
all other data protocols (such as VJ packets) in a generic way.

src/netif/ppp/ppp.c

@@ -768,14 +768,7 @@ void ppp_input(ppp_pcb *pcb, struct pbuf *pb) {
    * the protocol is in the range of what should be encrypted.
    * At the least, we drop this packet.
    */
-  if (pcb->settings.require_mppe && (0
-#if PPP_IPV4_SUPPORT
-       || protocol == PPP_IP
-#endif /* PPP_IPV4_SUPPORT */
-#if PPP_IPV6_SUPPORT
-       || protocol == PPP_IPV6
-#endif /* PPP_IPV6_SUPPORT */
-       )) {
+  if (pcb->settings.require_mppe && protocol != PPP_COMP && protocol < 0x8000) {
     PPPDEBUG(LOG_ERR, ("ppp_input[%d]: MPPE required, received unencrypted data!\n", pcb->netif->num));
     goto drop;
   }