cfg80211: add WMM traffic stream API

Add nl80211 and driver API to validate, add and delete traffic
streams with appropriate settings.

The API calls for userspace doing the action frame handshake
with the peer, and then allows only to set up the parameters
in the driver. To avoid setting up a session only to tear it
down again, the validate API is provided, but the real usage
later can still fail so userspace must be prepared for that.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>