mac80211: support active monitor interfaces

Support them only if the driver advertises support for them via
IEEE80211_HW_SUPPORTS_ACTIVE_MONITOR. Unlike normal monitor interfaces,
they are added to the driver, along with their MAC address.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>