ath9k: fix beacon slot/buffer leak

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>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>