Fix a missing initialisation of bt_flags when setting up the TDMA beacon.

The AR5212 HAL didn't check this field; timers are enabled a different
way.

The AR5416 HAL however did, and since this field was uninitialised, it had
whatever was on the stack at the time. This lead to "unpredictable"
behaviour.

This allows TDMA to work on the AR5416 and later chipsets.

Thanks to: paradyse@gmail.com
Approved by: re (kib, blanket)