mac80211: add support for "active" monitor interfaces which allow userspace tools to connect to APs via injection

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36743 3c298f89-4303-0410-b956-a3cf2f4a3e73