ath9k: Fix wow init/deinit

Registering the card as a wakeup source needs to
be done once, during initialization. When the WOW
configuration changes, the card's status as wakeup
source needs to be changed too and this is done
via the set_wakeup() callback. Also, make sure
the device is removed properly using ath9k_deinit_wow().

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>