mac80211_hwsim: Fix NULL pointer dereference

mac80211_hwsim was crashing when receiving tx information from user
space. Crash happens because txi->rate_driver_data[0] is pointing to a
non valid memory address.

This code path is only used by wmediumd and wmediumd doesn't provide
multiple channel support, so we can pass the channel struct
(data2->channel) directly to mac80211_hwsim_monitor_ack function.

Signed-off-by: Javier Lopez <jlopex@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>