refactor(uORB): drop redundant lock around the atomic _advertised store

DeviceNode::open() took the cdev lock only to call mark_as_advertised(), but
_advertised is a px4::atomic, so the single store needs no lock. (Suggested in
review.)

platforms/common/uORB/uORBDeviceNode.cpp

@@ -76,9 +76,8 @@ uORB::DeviceNode::open(cdev::file_t *filp)
 	/* is this a publisher? */
 	if (filp->f_oflags == PX4_F_WRONLY) {
 
-		lock();
+		// _advertised is atomic, so no lock is needed around this single store
 		mark_as_advertised();
-		unlock();
 
 		/* now complete the open */
 		return CDev::open(filp);