iwlwifi: mvm: don't send BAR on flushed frames

When we flush a queue, the packets will have a 'failed'
status but we shouldn't send a BAR. This check was missing.
Because of that, when we got an ampdu_action with
IEEE80211_AMPDU_TX_STOP_FLUSH, we started the following
ping pong with the firmware:

1) Set the station as 'draining'
2) Get a failed Tx status (DRAINED)
3) Send a BAR because of the failed Tx status

(loop of 2 and 3)

This loop wasn't endless since the BAR isn't sent on a
queue that would trigger a "nested" BAR.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>