wifi: move action code from per-type frame structs

wifi: move action code from per-type frame structs

The action code actually serves to identify the type of action
frame, so it really isn't part of the per-type structure. Pull
it out and have it in the general action frame format.

In theory, whether or not the action code is present in this
way is up to each category, but all categories that are defined
right now all have that value.

While at it, and since this change requires changing all users,
remove the 'u' and make it an anonymous union in this case, so
that all code using this changes.

Change IEEE80211_MIN_ACTION_SIZE to take an argument which says
how much of the frame is needed, e.g. category, action_code or
the specific frame type that's defined in the union. Again this
also ensures that all code is updated.

In some cases, fix bugs where the SKB length was checked after
having accessed beyond the checked length, in particular in FTM
code, e.g. ieee80211_is_ftm().

Link: https://patch.msgid.link/20260226183607.67e71846b59e.I9a24328e3ffcaae179466a935f1c3345029f9961@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>