mac80211: tracing: avoid 'idx' variable

This variable shadows something that gets generated inside
the tracing macros, which causes sparse to warn. Avoid it
so sparse output is more readable, even if it doesn't seem
to cause any trouble.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/trace.h

@@ -1052,10 +1052,10 @@ TRACE_EVENT(drv_ampdu_action,
 );
 
 TRACE_EVENT(drv_get_survey,
-	TP_PROTO(struct ieee80211_local *local, int idx,
+	TP_PROTO(struct ieee80211_local *local, int _idx,
 		 struct survey_info *survey),
 
-	TP_ARGS(local, idx, survey),
+	TP_ARGS(local, _idx, survey),
 
 	TP_STRUCT__entry(
 		LOCAL_ENTRY
@@ -1064,7 +1064,7 @@ TRACE_EVENT(drv_get_survey,
 
 	TP_fast_assign(
 		LOCAL_ASSIGN;
-		__entry->idx = idx;
+		__entry->idx = _idx;
 	),
 
 	TP_printk(