ath9k_htc: Fix WMI and beacon header

Match the beacon header with that of the firmware.
Also, the firmware reports the TSF for an SWBA, so
store it.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>