drivers/net/ppp*.c: fix sparse warnings: fix signedness

Fix this sparse warnings:
  drivers/net/ppp_generic.c:919:43: warning: incorrect type in argument 1 (different signedness)
  drivers/net/pppoe.c:1195:43: warning: incorrect type in argument 1 (different signedness)
  drivers/net/pppol2tp.c:2666:43: warning: incorrect type in argument 1 (different signedness)
Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ppp_generic.c

@@ -183,7 +183,7 @@ static atomic_t ppp_unit_count = ATOMIC_INIT(0);
 static atomic_t channel_count = ATOMIC_INIT(0);
 
 /* per-net private data for this module */
-static unsigned int ppp_net_id;
+static int ppp_net_id;
 struct ppp_net {
 	/* units to ppp mapping */
 	struct idr units_idr;

drivers/net/pppoe.c

@@ -97,7 +97,7 @@ static const struct proto_ops pppoe_ops;
 static struct ppp_channel_ops pppoe_chan_ops;
 
 /* per-net private data for this module */
-static unsigned int pppoe_net_id;
+static int pppoe_net_id;
 struct pppoe_net {
 	/*
 	 * we could use _single_ hash table for all

drivers/net/pppol2tp.c

@@ -232,7 +232,7 @@ static struct ppp_channel_ops pppol2tp_chan_ops = { pppol2tp_xmit , NULL };
 static struct proto_ops pppol2tp_ops;
 
 /* per-net private data for this module */
-static unsigned int pppol2tp_net_id;
+static int pppol2tp_net_id;
 struct pppol2tp_net {
 	struct list_head pppol2tp_tunnel_list;
 	rwlock_t pppol2tp_tunnel_list_lock;