mac80211: Recalc min chandef when station is associated

The minimum chandef bandwidth calculation was done only in case
a new station was inserted (or when an existing station was removed).
However, it is possible that stations are inserted before they are
associated, e.g., when FULL_AP_CLIENT_STATE is supported and user
space adds stations unassociated.

Fix this by calling ieee80211_recalc_min_chandef() whenever
a station transitions in/out the associated state, and only
consider station marked as associated.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>