cfg80211: clarify frames covered by average ACK signal report

Modify the API to include all ACK frames in average ACK
signal strength reporting, not just ACKs for data frames.
Make exposing the data conditional on implementing the
extended feature flag.

This is how it was really implemented in mac80211, update
the code there to use the new defines and clean up some of
the setting code.

Keep nl80211.h source compatibility by keeping the old names.

Signed-off-by: Balaji Pothunoori <bpothuno@codeaurora.org>
[rewrite commit log, change compatibility to be old=new
 instead of the other way around, update kernel-doc,
 roll in mac80211 changes, make mac80211 depend on valid
 bit instead of HW flag]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>