sink: document the frame-size divisor in free-frames

Free-frames divides the free size by the frame size, which is the channel
count times the sample size. A zero divisor was the concern, as the
channel count is host-supplied; it is now rejected at module init
(module_adapter_ipc4.c) before it reaches the stream, and the sample size
is fixed by a valid frame format.

Document that invariant here rather than re-checking it on this hot path.

Signed-off-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>

Author: lrgirdwo

src/module/audio/sink_api.c

@@ -109,6 +109,11 @@ EXPORT_SYMBOL(sink_get_frame_bytes);
 
 size_t sink_get_free_frames(struct sof_sink *sink)
 {
+	/* The frame size is a valid divisor: a host channel count of zero is
+	 * rejected at module init (module_adapter_ipc4.c) before it reaches
+	 * the stream, and the sample size is fixed by a valid frame format, so
+	 * this is not re-checked on the hot path.
+	 */
 	return sink_get_free_size(sink) / sink_get_frame_bytes(sink);
 }
 EXPORT_SYMBOL(sink_get_free_frames);