After the corresponding mac80211 patch, we can now report the airtime
used for each transmitted packet and mac80211 will be able to implement
WMM-AC with that information.

To support WMM-AC in the driver then, report the airtime and advertise
support.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>