LinuxKPI: 802.11: ieee80211_vif_usable_links()

ieee80211_vif_usable_links() shows up in TODO() tracking.
Turns out the return (1) is likely wrong and should be 0 given we do
not support MLD yet. For now return the valid_links bitmap and leave
an IMPROVE as likely not all of the valid ones will be useable.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days