ath9k: fix beacon slot/buffer leak

ath9k: fix beacon slot/buffer leak

commit 74401773f80b6d42f7a4c6994ca0cca883b03745 upstream.

When cleaning up beacon buffers and slots, ath9k currently checks if
sc->ah->opmode is set to a beacon related mode before cleaning up
buffers.
An unfortunate ordering of interface up/down commands can lead to
sc->ah->opmode being set to monitor mode, while there are AP interfaces
present on the same wiphy.
Always cleaning up beacon buffers if present fixes this issue.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>