wifi: mac80211: reorg some iface data structs for MLD

Start reorganizing interface related data structures toward
MLD. The most complex part here is for the keys, since we
have to split the various kinds of GTKs off to the link but
still need to use (for WEP) the other keys as a fallback
even for multicast frames.

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