ath9k: Special processing for channel changes during scan

Allow mac80211-controlled channel changes on an active wiphy and
especially during a scan. We need this as long as the scan is
controlled by mac80211. Moving this control into the driver could
allow some optimizations on scanning while using multiple virtual
interfaces, but for now, try to work as well as possible with the
current scan mechanism.

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