filters: use filter_Flush() instead of accessing the callback directly

It's easier to find out who flushes filters and when.

It's also easier to reorder the code without having the Flush callback above a
lot of common code.

The di_flush call in MMAL passthrough mode was probably wrong.

modules/audio_filter/converter/tospdif.c

@@ -588,7 +588,7 @@ static block_t *DoWork( filter_t *p_filter, block_t *p_in_buf )
         case SPDIF_MORE_DATA:
             break;
         case SPDIF_ERROR:
-            Flush( p_filter );
+            filter_Flush( p_filter );
             break;
     }
 

modules/hw/mmal/converter.c

@@ -711,7 +711,7 @@ fail:
         mmal_buffer_header_release(out_buf);
     if (p_pic != NULL)
         picture_Release(p_pic);
-    conv_flush(p_filter);
+    filter_Flush(p_filter);
     return NULL;
 }