Simplify refresh_frame_flags inference on S_FRAME

Accordingly to design doc(R19), refresh_frame_frame_flags must be set to 0xFF.

Change-Id: I235b12a2196586cbdd6801ed9959a095b135917a

av1/decoder/decodeframe.c

diff --git a/av1/decoder/decodeframe.c b/av1/decoder/decodeframe.c
index aaeebb0..ddd7825 100644
--- a/av1/decoder/decodeframe.c
+++ b/av1/decoder/decodeframe.c
@@ -2856,7 +2856,7 @@
     } else if (pbi->need_resync != 1) { /* Skip if need resync */
 #if CONFIG_OBU
       pbi->refresh_frame_flags = (cm->frame_type == S_FRAME)
-                                     ? ~(1 << REF_FRAMES)
+                                     ? 0xFF
                                      : aom_rb_read_literal(rb, REF_FRAMES);
 #else
       pbi->refresh_frame_flags = aom_rb_read_literal(rb, REF_FRAMES);