ath9k_hw: add multicast key search support

If a MAC address for a key table entry is flagged with the
multicast bit (0x01), indicate to the hardware that multicast
lookup instead of unicast lookup should be used. The multicast
bit itself never makes it to the actual keytable entry register,
as it is shifted out.

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