pcm: Make more of the low-level PCM API private

* pcm_play_data
 * pcm_play_stop
 * pcm_play_stop_int
 * pcm_is_playing
 * pcm_set_frequency
 * pcm_get_frequency
 * pcm_apply_settings

Now, the only user of these functions are the mixer and recording layers
that provide a higher-level API to plugins and the main [playback]
application.

Outside of the PCM core, pcm_apply_settings() was only used immediately
following a call to mixer_set_frequency(), so the latter function
now always calls the former.

Change-Id: I61c3144dc156b9de9b7963160b525c6d10c6ad4b

apps/plugin.c

@@ -644,7 +644,6 @@ static const struct plugin_api rockbox_api = {
 #endif
     &audio_master_sampr_list[0],
     &hw_freq_sampr[0],
-    pcm_apply_settings,
     pcm_play_lock,
     pcm_play_unlock,
     pcm_current_sink_caps,