ath10k: make sure to not leak beacon dma mapping

If for some reason mac80211 wouldn't stop
beaconing gracefully and just removed interface of
a running AP/IBSS interface it was possible to
leak pending beacon DMA mapping. It's very
unlikely but better safe than sorry.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>