iwlwifi: disable aggregation queue if stopped early

When aggregation is stopped again for some reason
before the queue we selected has drained, we will
currently leak the TX queue and keep it enabled
for aggregation. Normally this doesn't happen, so
the problem is rarely seen.

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>