mac80211: fix AP/VLAN PS buffer race

When an AP interface is removed without the
AP/VLAN interfaces having been removed before
already, the AP-VLAN interface might still
have sleeping stations and buffer multicast
frames which will happen on the AP interface.
Thus, we need to remove AP/VLAN interfaces
before purging buffered broadcast frames.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>