iwlwifi: make software queue assignment more efficient

There really is no reason to be assigning txq->swq_id all the
time, once at aggregation setup is sufficient.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>