mac80211: lock sdata in ieee80211_csa_connection_drop_work()

We call ieee80211_ibss_disconnect(), which requires sdata to be
locked, so lock the sdata during ieee80211_csa_connection_drop_work().

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