sched: act: ife: move encode/decode check to init

This patch adds the check of the two possible ife handlings encode
and decode to the init callback. The decode value is for usability
aspect and used in userspace code only. The current code offers encode
else decode only. This patch avoids any other option than this.

Signed-off-by: Alexander Aring <aring@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>