mac80211: move csa_active setting in STA CSA

The sdata->vif.csa_active could be left set after,
e.g. channel context constraints check fail in STA
mode leaving the interface in a strange state for
a brief period of time until it is disconnected.
This was harmless but ugly.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Reviewed-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>