audio_format: changed "bits" to "enum sample_format"

This patch prepares support for floating point samples (and probably
other formats).  It changes the meaning of the "bits" attribute from a
bit count to a symbolic value.

src/encoder/lame_encoder.c

@@ -185,7 +185,7 @@ lame_encoder_open(struct encoder *_encoder, struct audio_format *audio_format,
 {
 	struct lame_encoder *encoder = (struct lame_encoder *)_encoder;
 
-	audio_format->bits = 16;
+	audio_format->format = SAMPLE_FORMAT_S16;
 	audio_format->channels = 2;
 
 	encoder->audio_format = *audio_format;