LinuxKPI: 802.11: improve assertion and tkip code

Move a KASSERT out of a function and make it a CTASSERT with
appropriate comments.

Skeleton implement two tkip functions, still left TODO, initializing
variables with dummy values to quiten compiler warnings. It is
unclear to me if we should still ever properly implement TKIP
compat code at this point. If so the current code gives a good
idea what needs to be done in addition to allocating references
to real state along with keyconf.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days