skip WOW if no interfaces are up

Change-Id: I5c1e7311d2c1317af62b5c93b89e32a27d6a87dd
ath6kl_wow_suspend: avoid deadlock in ath6kl_wow_suspend_vif

Change-Id: I2e6889876b4451c2d6016abd5a5f5e4615d1fa81
ath6kl_wow_resume: avoid deadlock in ath6kl_wow_resume_vif

Change-Id: I00ecb17e751b0791c100142b06c0cc62e87537e7