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>