PPP, PPPoS moved fd from ppp_pcb to pppos_pcb

Moved fd from ppp_pcb to pppos_pcb.

However PPP ioctl PPPCTLG_FD is temporarily disabled until I found a
better solution.

src/include/netif/ppp/ppp.h

@@ -352,9 +352,6 @@ struct ppp_pcb_s {
   u8_t num;                      /* Interface number - only useful for debugging */
 #endif /* PPP_DEBUG */
   ppp_settings settings;
-#if PPPOS_SUPPORT
-  sio_fd_t fd;                   /* File device ID of port. */
-#endif /* PPPOS_SUPPORT */
   link_command_cb_fn link_command_cb;
   link_write_cb_fn link_write_cb;
   link_netif_output_cb_fn link_netif_output_cb;

src/include/netif/ppp/pppos.h

@@ -44,7 +44,8 @@
  */
 typedef struct pppos_pcb_s pppos_pcb;
 struct pppos_pcb_s {
-  ppp_pcb *ppp;                /* PPP PCB */
+  ppp_pcb *ppp;                  /* PPP PCB */
+  sio_fd_t fd;                   /* File device ID of port. */
 };
 
 /* Create a new PPPoS session. */