net80211: wakeup scan_start() task when null data frame was actually
- Use M_TXCB mechanism to report about null data frame transmission.
- Increase timeout from 1 to 10 ms (the previous one may be not enough
for non-empty queue).
- Intel 3945BG, STA mode.
- RTL8188CUS, STA mode.
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D5147