mac80211: notify the driver when a sta uses 4-address mode

This is needed for encapsulation offload of 4-address mode packets

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20200908123702.88454-14-nbd@nbd.name


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