rawmidi: use snd_dlobj_cache_get2() in rawmidi open (coverity)

Use proper reference counting for the dynamic symbol.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>

src/rawmidi/rawmidi_local.h

@@ -37,7 +37,7 @@ typedef struct {
 } snd_rawmidi_ops_t;
 
 struct _snd_rawmidi {
-	void *dl_handle;
+	void *open_func;
 	char *name;
 	snd_rawmidi_type_t type;
 	snd_rawmidi_stream_t stream;