avfilter/af_volume: Change enums to int, which are accessed via AVOption as int

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

libavfilter/af_volume.h

@@ -68,13 +68,13 @@ enum ReplayGainType {
 typedef struct VolumeContext {
     const AVClass *class;
     AVFloatDSPContext *fdsp;
-    enum PrecisionType precision;
-    enum EvalMode eval_mode;
+    int precision;
+    int eval_mode;
     const char *volume_expr;
     AVExpr *volume_pexpr;
     double var_values[VAR_VARS_NB];
 
-    enum ReplayGainType replaygain;
+    int replaygain;
     double replaygain_preamp;
     int    replaygain_noclip;
     double volume;