orb: add orb_get_interval to API

src/modules/uORB/uORBDevices_nuttx.cpp

@@ -311,6 +311,10 @@ uORB::DeviceNode::ioctl(struct file *filp, int cmd, unsigned long arg)
 		//and only one advertiser is allowed to open the DeviceNode at the same time.
 		return update_queue_size(arg);
 
+	case ORBIOCGETINTERVAL:
+		*(unsigned *)arg = sd->update_interval;
+		return OK;
+
 	default:
 		/* give it to the superclass */
 		return CDev::ioctl(filp, cmd, arg);