While the change between 802.11-2012 and 802.11-2016 to move from
requiring APs to set the two top bits to now requiring them to be
cleared was apparently unintentional and will be fixed, clients
should either way assume that the top five bits are reserved and
ignore them.

Implement that in mac80211.
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>