iwlwifi: mvm: fix aggregation drain flow

Move the counter for non-AMPDU frames to mvm. It is needed
for the drain flow which happens once the ieee80211_sta has
been freed, so keeping it in iwl_mvm_sta which is embed into
ieee80211_sta is not a good idea.

Also, since its purpose it to remove the STA in the fw only
after all the frames for this station have exited the shared
Tx queues, we need to decrement it in the reclaim flow. This
flow can happen after ieee80211_sta has been removed, which
means that we have no iwl_mvm_sta there. So we can't know
what is the vif type. Hence, we know audit these frames for
all the vif types.
In order to avoid spawning sta_drained_wk all the time, we
now check that we are in a flow in which draining might
happen - only when mvmsta is NULL. This is better than
previous code that would spawn sta_drained_wk all the time
in AP mode.

Cc: stable@vger.kernel.org [3.9]
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Reviewed-by: Ilan Peer <ilan.peer@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>

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