mac80211: fix nested sdata lock for IBSS/CSA

This fixes a regression introduced by my patch "mac80211: don't cancel
csa finalize work within stop_ap", which added sdata locks to
ieee80211_csa_finalize_work() without removing the locking for
ieee80211_ibss_finish_csa(), which is called by the former, resulting
in a deadlock due to nested locking.

Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>