LinuxKPI: 802.11: add tunable to enable TKIP

On one hand TKIP support for other drivers or older iwlwifi chipsets
likely needs more work (see mac80211.h), on the other hand we should
no longer use TKIP (but we understand people still need to).

Add a tunable to enable it for those who want to test it with modern
iwlwifi chipsets. hw_crypto also still needs to be enabled.

I could only test it with legacy rates as none of my (non-FreeBSD)
APs allowed me by default to use TKIP anymore (had to force it on)
and then they would not let me associate with HT or VHT at all if
TKIP was on.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days