ath9k: update hw_timer_enabled to false when we stop generic timers

Update the 'hw_timer_enabled' to 'false' wherever we are stopping
hardware generic timers, excecpt the case where we start them
again immediately.

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>