ath9k: use software queueing for multicast traffic

Create a per-vif dummy node entry for keeping the multicast software
queues. This helps in setups with a lot of mulitcast traffic that could
otherwise potentially drown out unicast traffic to stations.

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