mac80211_hwsim: Fix radiotap header for ACK frames

The earlier addition of rt_tsft to struct hwsim_radiotap_hdr updated
only mac80211_hwsim_monitor_tx() to fill in the new field.
mac80211_hwsim_monitor_ack() did not set the rt_tsft field and as such,
leaked eight bytes of kernel memory to user space. In addition, the
resulting radiotap header is invalid since the field offsets do not
match. Fix these issues by defining a separate radiotap header structure
for the ACK frame case which does not use all the fields.

Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>