mac80211_hwsim: Send ACK frames on the hwsim0 interface

Report successful transmissions (receiver awake and on the same
channel) by generating ACK frames on the hwsim0 interface. This makes
it easier to figure out from packet capture logs whether frames were
delivered or not.

Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>