cfg80211: handle add_station auth/assoc flag quirks

When a new station is added to AP/GO interfaces the default behaviour
is for it to be added authenticated and associated, due to backwards
compatibility. To prevent that, the driver must be able to do that
(setting the NL80211_FEATURE_FULL_AP_CLIENT_STATE feature flag) and
userspace must set the flag mask to auth|assoc and clear the set.

Handle this quirk in the API entirely in nl80211, and always push the
full flags to the drivers. NL80211_FEATURE_FULL_AP_CLIENT_STATE is
still required for userspace to be allowed to set the mask including
those bits, but after checking that add both flags to the mask and
set in case userspace didn't set them otherwise.

This obsoletes the mac80211 code handling this difference, no other
driver is currently using these flags.

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