Revert "Subscription remove unused instance class member"

This reverts commit cf2d794da9.

src/modules/uORB/Subscription.cpp

@@ -43,7 +43,8 @@ namespace uORB
 {
 
 SubscriptionBase::SubscriptionBase(const struct orb_metadata *meta, unsigned interval, unsigned instance) :
-	_meta(meta)
+	_meta(meta),
+	_instance(instance)
 {
 	if (instance > 0) {
 		_handle = orb_subscribe_multi(_meta, instance);