cfg80211: allow changing station capabilities for unassociated stations

Currently, cfg80211 rejects capability updates for existing entries
and as a result it's impossible to update entries that were added
unassociated, but that is necessary to go through the full station
states from userspace, adding a station before authentication etc.

Fix this by allowing updates to capabilities for stations that the
driver (or mac80211) assigned unassociated state. Drivers setting
the full station state support flag must use the new station type
for proper operation.

Signed-off-by: Ayala Beker <ayala.beker@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>