[PKT_SCHED]: GRED: Do not reset statistics in gred_reset/gred_change

Qdiscs are not supposed to reset statistics in reset() and while
changing parameters. My argumentation is that if the user wants
the counters to be reset he can simply remove and readd the
qdiscs, that's what most users do anyway.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>