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.

test/run_output.c

@@ -119,7 +119,7 @@ int main(int argc, char **argv)
 		return 1;
 	}
 
-	audio_format_init(&audio_format, 44100, 16, 2);
+	audio_format_init(&audio_format, 44100, SAMPLE_FORMAT_S16, 2);
 
 	g_thread_init(NULL);