ath: Export ath_hw_keysetmac()

ath9k is going to use this for safer management of key cache entries.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201214172118.18100-4-jouni@codeaurora.org