uORBDevices: set the priority when advertising a node that's already subscribed

This fixes the case where a topic instance is already subscribed, and
advertised later. The subscriber will create the DeviceNode with default
priority, and the advertiser will just use the existing DeviceNode,
without updating to the requested priority.

src/modules/uORB/uORBDevices.cpp

@@ -927,6 +927,7 @@ uORB::DeviceMaster::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 
 						if ((existing_node != nullptr) && !(existing_node->is_published())) {
 							/* nothing has been published yet, lets claim it */
+							existing_node->set_priority(adv->priority);
 							ret = PX4_OK;
 
 						} else {