ath9k: Remove redundant device_can_wakeup() check

WOW capability is registered with mac80211 only when
the device has the ability to wakeup, so there is no
need to check in the suspend() routine.

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