mac80211_hwsim: fix race condition with sta/vif pointers

info->control.sta and control.vif may only be dereferenced
during the drv_tx call otherwise could lead to use-after-free
bugs.

Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
[reword commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>