mac80211_hwsim: Set IEEE80211_STAT_ACK flag when userspace indicates that the frame has been acknowledged.

The station fail average is not updated correctly since the
IEEE80211_STAT_ACK flag is not set when using wmediumd with
mac80211_hwsim. Set this flag when wmediumd indicates that the frame
was successfully transmitted (eventually).
Signed-off-by: Qasim Javed <qasimj@gmail.com>
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>