mac80211: respect rate mask in TX

Bitrate mask were not respected in transmissions, causing (for
example) P2P GO/client to use CCK rates for auth and assoc frames.
Fix it by considering the rate mask in __rate_control_send_low().

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>