avformat/spdifenc: fix TrueHD streams over 48kHz

Commit 36e156bef0 ("avformat/spdifenc: fix handling of large TrueHD
frame") added an obviously incorrect bitshift that caused incorrect
samples-per-frame calculation for TrueHD streams over 48kHz.

Fix that.

libavformat/spdifenc.c

@@ -435,9 +435,9 @@ static int spdif_header_truehd(AVFormatContext *s, AVPacket *pkt)
     if (AV_RB24(pkt->data + 4) == 0xf8726f) {
         /* major sync unit, fetch sample rate */
         if (pkt->data[7] == 0xba)
-            ratebits = pkt->data[8] >> 8;
+            ratebits = pkt->data[8] >> 4;
         else if (pkt->data[7] == 0xbb)
-            ratebits = pkt->data[9] >> 8;
+            ratebits = pkt->data[9] >> 4;
         else
             return AVERROR_INVALIDDATA;