audio_format: added audio_format_frame_size()

A frame contains one sample per channel, thus it is sample_size *
channels.  This patch includes some cleanup for various locations
where the sample size for 24 bit audio was still 3 bytes (instead of
4).

src/timer.c

@@ -40,7 +40,7 @@ Timer *timer_new(const struct audio_format *af)
 	timer = xmalloc(sizeof(Timer));
 	timer->time = 0;
 	timer->started = 0;
-	timer->rate = af->sample_rate * (af->bits / CHAR_BIT) * af->channels;
+	timer->rate = af->sample_rate * audio_format_frame_size(af);
 
 	return timer;
 }