mac80211: unify CSA action frame/beacon processing

CSA action frame content should be processed as variable IEs
rather than fixed to make it extensible. Unify the code and
process them just like CSA in beacons to make it easier to
extend for HT/VHT.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/linux/ieee80211.h

@@ -840,9 +840,7 @@ struct ieee80211_mgmt {
 				} __packed wme_action;
 				struct{
 					u8 action_code;
-					u8 element_id;
-					u8 length;
-					struct ieee80211_channel_sw_ie sw_elem;
+					u8 variable[0];
 				} __packed chan_switch;
 				struct{
 					u8 action_code;