iwlwifi: remove un-necessary rs_tl_turn_on_agg() after agg enabled

After the MLME handshaking complete and tx aggregation started for the
tid. Do not send unnecessary turn on aggregation request to mac80211.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>