bluetooth: Add device pointer to transport

Transports always have an associated device, so add the pointer as a
member to the structure, and remove the discovery pointer since it
already exists in the device object.

src/modules/bluetooth/bluetooth-util.h

@@ -71,7 +71,7 @@ typedef enum pa_bluetooth_transport_hook {
 } pa_bluetooth_transport_hook_t;
 
 struct pa_bluetooth_transport {
-    pa_bluetooth_discovery *y;
+    pa_bluetooth_device *device;
     char *owner;
     char *path;
     enum profile profile;