avfilter/vf_phase: Change enum to int, which is accessed via AVOption as int

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

libavfilter/vf_phase.c

@@ -41,7 +41,7 @@ enum PhaseMode {
 
 typedef struct PhaseContext {
     const AVClass *class;
-    enum PhaseMode mode;
+    int mode;                   ///<PhaseMode
     AVFrame *frame; /* previous frame */
     int nb_planes;
     int planeheight[4];