ath9k: Virtual wiphy pause/unpause functionality

Allow virtual wiphys to be paused/unpaused to allow off-channel
operations. Pause will stop all TX queues for the wiphy and move the
STA into power save mode if in managed mode. Unpause wakes up the TX
queues and notifies the AP that the STA woke up if in managed mode.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>