ath9k: Check WOW triggers properly

This patch makes sure that valid WOW triggers
are present before trying to suspend the device.
Also, introduce and use ATH_OP_WOW_ENABLED to
bypass PCI suspend and clear it in resume().

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