ALSA: compress: Add SNDRV_PCM_STATE_PREPARED state explanation

Stream states were explained in the code comments but
SNDRV_PCM_STATE_PREPARED was missed so add it

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/compress_offload.c

@@ -74,6 +74,9 @@ struct snd_compr_file {
  * SNDRV_PCM_STATE_SETUP: When stream has been initialized. This is done by
  *	calling SNDRV_COMPRESS_SET_PARAMS. running streams will come to this
  *	state at stop by calling SNDRV_COMPRESS_STOP, or at end of drain.
+ * SNDRV_PCM_STATE_PREPARED: When a stream has been written to (for
+ *	playback only). User after setting up stream writes the data buffer
+ *	before starting the stream.
  * SNDRV_PCM_STATE_RUNNING: When stream has been started and is
  *	decoding/encoding and rendering/capturing data.
  * SNDRV_PCM_STATE_DRAINING: When stream is draining current data. This is done