ath9k: set beacon related ps flags on bss_info change

Requesting beacon sync up to configure beacon timers properly
in hw, has be done after doing beacon config with default values.
Setting the flags in beacon config is causing the device to not
enter into network sleep on idle state.

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