ath6kl: Add PMKSA cache management commands

Implement cfg80211_ops set_pmksa, del_pmksa, and flush_pmksa to enable
PMKSA caching support.

kvalo: backported from ath6kl-cleanup tree

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>