mac80211: fix handling TIM IE when stations disconnect

When a station disconnects with frames still pending, we clear
the TIM bit, but too late - it's only cleared when the station
is already removed from the driver, and thus the driver can get
confused (and hwsim will loudly complain.)

Fix this by clearing the TIM bit earlier, when the station has
been unlinked but not removed from the driver yet. To do this,
refactor the TIM recalculation to in that case ignore traffic
and simply assume no pending traffic - this is correct for the
disconnected station even though the frames haven't been freed
yet at that point.

This patch isn't needed for current drivers though as they don't
check the station argument to the set_tim() operation and thus
don't really run into the possible confusion.

Reported-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>