wifi: cfg80211: split struct cfg80211_ap_settings

Using the full struct cfg80211_ap_settings for an update is
misleading, since most settings cannot be updated. Split the
update case off into a new struct cfg80211_ap_update.

Change-Id: I3ba4dd9280938ab41252f145227a7005edf327e4
Signed-off-by: Johannes Berg <johannes.berg@intel.com>