ath9k_common: use allocated key cache entries for multi BSS crypto support

This patch replaces the buggy 'ath9k: Group Key fix for VAPs' change.

For AP mode group keys, use the BSSID as lookup mac address, with
the multicast keysearch bit set.
For IBSS mode, use the peer's MAC address with multicast keysearch.
For STA mode, keep using the group key slots.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>