mac80211: set default QoS values according to spec

We've never really cared about the default QoS (WMM) values, but
we really should if the AP doesn't send any. This patch makes
mac80211 use the default values according to 802.11-2007, and
additionally syncs the default values when we disassociate so
whatever the last AP said gets "unconfigured".

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>