cfg80211: clarify the requirements of .disconnect()

cfg80211 expects the .disconnect() handler to call
cfg80211_disconnect() when done. Make this requirement
more explicit.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>