cfg80211: Add airtime statistics and settings

This adds TX airtime statistics to the cfg80211 station dump (to go along
with the RX info already present), and adds a new parameter to set the
airtime weight of each station. The latter allows userspace to implement
policies for different stations by varying their weights.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
[rmanohar@codeaurora.org: fixed checkpatch warnings]
Signed-off-by: Rajkumar Manoharan <rmanohar@codeaurora.org>
[move airtime weight != 0 check into policy]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>