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/faad_plugin.c

@@ -283,7 +283,7 @@ faad_decoder_init(faacDecHandle decoder, struct decoder_buffer *buffer,
 	decoder_buffer_consume(buffer, nbytes);
 
 	return audio_format_init_checked(audio_format, sample_rate,
-					 16, channels, error_r);
+					 SAMPLE_FORMAT_S16, channels, error_r);
 }
 
 /**