lavd/alsa: set frame_size field.

The value tells the typical size of a packet from the demuxer.

libavdevice/alsa-audio-dec.c

@@ -80,6 +80,7 @@ static av_cold int audio_read_header(AVFormatContext *s1)
     st->codec->codec_id    = codec_id;
     st->codec->sample_rate = s->sample_rate;
     st->codec->channels    = s->channels;
+    st->codec->frame_size = s->frame_size;
     avpriv_set_pts_info(st, 64, 1, 1000000);  /* 64 bits pts in us */
     /* microseconds instead of seconds, MHz instead of Hz */
     s->timefilter = ff_timefilter_new(1000000.0 / s->sample_rate,