wifi: mac80211: optionally implement MLO multicast TX

For drivers using software encryption for multicast TX, such
as mac80211_hwsim, mac80211 needs to duplicate the multicast
frames on each link, if MLO is enabled. Do this, but don't
just make it dependent on the key but provide a separate flag
for drivers to opt out of this.

This is not very efficient, I expect that drivers will do it
in firmware/hardware or at least with DMA engine assistence,
so this is mostly for hwsim.

To make this work, also implement the SNS11 sequence number
space that an AP MLD shall have, and modify the API to the
__ieee80211_subif_start_xmit() function to always require the
link ID bits to be set.

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