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>
diff --git a/drivers/net/pppol2tp.c b/drivers/net/pppol2tp.c
index 1ba0f68..5b07dd8 100644
--- a/drivers/net/pppol2tp.c
+++ b/drivers/net/pppol2tp.c
@@ -232,7 +232,7 @@
 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;