ath9k: Fix beacon reconfiguration

When a reset or a channel-change happens, for managed mode,
the HW beacon timers have to be programmed after the TSF has
been synchronized. This is handled via the sync flags.

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