mac80211: handle aggregation session timeout on fast-xmit path

The conversion to the fast-xmit path lost proper aggregation session
timeout handling - the last_tx wasn't set on that path and the timer
would therefore incorrectly tear down the session periodically (with
those drivers/rate control algorithms that have a timeout.)

In case of iwlwifi, this was every 5 seconds and caused significant
throughput degradation.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>