avfilter/af_biquads: Change width_type to int as its accessed as int via AVOptions

This fixes depending on implementation defined behavior
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/af_biquads.c

@@ -98,7 +98,7 @@ typedef struct {
     const AVClass *class;
 
     enum FilterType filter_type;
-    enum WidthType width_type;
+    int width_type;
     int poles;
     int csg;