If the driver advertised support for a CAB queue, then we
should put all multicast frames there, otherwise sending
them can be racy with clients going to sleep while we TX
a frame. To avoid this, always TX multicast frames on the
multicast queue.

It seems like even drivers not using the queue framework
might want to do this which would mean also moving the
IEEE80211_TX_CTL_SEND_AFTER_DTIM flag assignment, but it
also seems that drivers behave differently here so that
just moving it wouldn't be a good idea. It'd be better to
modify those drivers to use the queue framework.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>