mac80211: Process multicast RX registration for Action frames

Convert a user space registration for processing multicast Action frames
(NL80211_CMD_REGISTER_FRAME with NL80211_ATTR_RECEIVE_MULTICAST) to a
new enum ieee80211_filter_flags bit FIF_MCAST_ACTION so that drivers can
update their RX filter parameters appropriately, if needed.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Link: https://lore.kernel.org/r/20200421144815.19175-1-jouni@codeaurora.org
[rename variables to rx_mcast_action_reg indicating action frames only]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>