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>
(cherry picked from http://mid.gmane.org/1390227670-19030-4-git-send-email-michal.kazior@tieto.com)