cfg80211: disallow interfering with stations on non-AP (part 2)

On non-AP interfaces userspace has no business interfering with
the station management, this can confuse mac80211 (and other
drivers probably wouldn't support it anyway). Allow adding and
removing stations only on AP interfaces.

(Reconcile this w/ previous version of patch posted with same
subject... -- JWL)

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>