iwlwifi: mvm: support IEEE80211_AMPDU_TX_STOP_FLUSH

mac80211 tells us when we need to dump the frames from the
AGG queue instead of releasing them as single MPDUs.
Being able to differentiate between the different cases
(IEEE80211_AMPDU_TX_STOP_*) allows us to handle races better.

When the station is removed, mac80211 asks to flush and
removes the station right away.
This allows to avoid a case where we still have frames in
AGG queues, but the station has been remove already.

Note that we can have frames on the shared queues, but this
is not a problem: the station in the fw will be kept until
all the frames on the shared queues have been drained.
AGG queues are a special case since they are dynamically
allocated.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>