filter/volume: don't use volume_level_get()

Added public methods to get and set the current volume.

test/run_filter.c

@@ -21,7 +21,6 @@
 #include "audio_parser.h"
 #include "audio_format.h"
 #include "filter_plugin.h"
-#include "volume.h"
 #include "pcm_volume.h"
 
 #include <glib.h>
@@ -31,11 +30,6 @@
 #include <errno.h>
 #include <unistd.h>
 
-int volume_level_get(void)
-{
-	return PCM_VOLUME_1;
-}
-
 static const struct config_param *
 find_named_config_block(const char *block, const char *name)
 {