ath9k: Postpone key cache entry deletion for TXQ frames reference it

Do not delete a key cache entry that is still being referenced by
pending frames in TXQs. This avoids reuse of the key cache entry while a
frame might still be transmitted using it.

To avoid having to do any additional operations during the main TX path
operations, track pending key cache entries in a new bitmap and check
whether any pending entries can be deleted before every new key
add/remove operation. Also clear any remaining entries when stopping the
interface.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201214172118.18100-6-jouni@codeaurora.org