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/decoder/sndfile_decoder_plugin.c

@@ -130,7 +130,8 @@ sndfile_stream_decode(struct decoder *decoder, struct input_stream *is)
 	/* for now, always read 32 bit samples.  Later, we could lower
 	   MPD's CPU usage by reading 16 bit samples with
 	   sf_readf_short() on low-quality source files. */
-	if (!audio_format_init_checked(&audio_format, info.samplerate, 32,
+	if (!audio_format_init_checked(&audio_format, info.samplerate,
+				       SAMPLE_FORMAT_S32,
 				       info.channels, &error)) {
 		g_warning("%s", error->message);
 		g_error_free(error);