ath: Use safer key clearing with key cache entries

It is possible for there to be pending frames in TXQs with a reference
to the key cache entry that is being deleted. If such a key cache entry
is cleared, those pending frame in TXQ might get transmitted without
proper encryption. It is safer to leave the previously used key into the
key cache in such cases. Instead, only clear the MAC address to prevent
RX processing from using this key cache entry.

This is needed in particularly in AP mode where the TXQs cannot be
flushed on station disconnection. This change alone may not be able to
address all cases where the key cache entry might get reused for other
purposes immediately (the key cache entry should be released for reuse
only once the TXQs do not have any remaining references to them), but
this makes it less likely to get unprotected frames and the more
complete changes may end up being significantly more complex.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201214172118.18100-2-jouni@codeaurora.org