iwlwifi: put use_rts_for_aggregation into hw_params

The hardware config ht_params shouldn't be modified,
so copy the use_rts_for_aggregation parameter into
hw_params and use/modify it there.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>