ath9k: Pause other virtual wiphys on channel change

For now, allow channel changes immediately and just force the other
virtual wiphys to paused state. This is needed to allow
mac80211-controlled scan to control channel changes.

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