ISDN/PPP: Reference struct ipppd directly

Instead of just recording the slot number and then retrieving
the ipppd via that, we can now just keep a pointer and get a
reference which makes sure that it does not go away until
we drop the reference.

drivers/isdn/i4l/isdn_net_lib.c

@@ -369,7 +369,7 @@ isdn_net_addif(char *name, isdn_net_local *mlp)
 	idev->pre_channel = -1;
 	idev->exclusive = -1;
 
-	idev->ppp_slot = -1;
+	idev->ipppd = NULL;
 	idev->pppbind = -1;
 
 	init_timer(&idev->dial_timer);

include/linux/isdn.h

@@ -410,7 +410,7 @@ typedef struct isdn_net_dev_s {
   int                    chargeint;    /* Interval between charge-infos    */
 
   int                    pppbind;      /* ippp device for bindings         */
-  int			 ppp_slot;     /* PPPD device slot number          */
+  struct ipppd          *ipppd;        /* /dev/ipppX which controls us     */
 
   struct sk_buff_head    super_tx_queue; /* List of supervisory frames to  */
 	                               /* be transmitted asap              */