net: sched: gred: separate error and non-error path in gred_change()

We will soon want to add more code to the non-error path, separate
it from the error handling flow.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>