ath9k: Clear key cache explicitly on disabling hardware

Now that ath/key.c may not be explicitly clearing keys from the key
cache, clear all key cache entries when disabling hardware to make sure
no keys are left behind beyond this point.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201214172118.18100-3-jouni@codeaurora.org