ath9k: add NL80211_EXT_FEATURE_CAN_REPLACE_PTK0 support

The ath9k driver was so far only able to rekey PTK0 keys correctly due
to the best effort queue flush added with commit 62872a9b


("mac80211: Fix PTK rekey freezes and clear text leak").

Add the needed queue flush and set NL80211_EXT_FEATURE_CAN_REPLACE_PTK0
to tell mac80211 that the driver can now rekey PTK0 keys correctly and
no longer needs the best effort flush.

Effectively this prevents mac80211 to warn when rekeying a PTK0 key
only.

Signed-off-by: Alexander Wetzel <alexander@wetzel-home.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200804164152.175375-1-alexander@wetzel-home.de