ath: Modify ath_key_delete() to not need full key entry

tkip_keymap can be used internally to avoid the reference to key->cipher
and with this, only the key index value itself is needed. This allows
ath_key_delete() call to be postponed to be handled after the upper
layer STA and key entry have already been removed. This is needed to
make ath9k key cache management safer.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201214172118.18100-5-jouni@codeaurora.org