mavlink_orb_subscription: minor optimization and comment fix

src/modules/mavlink/mavlink_orb_subscription.cpp

@@ -98,6 +98,10 @@ MavlinkOrbSubscription::update(const hrt_abstime t)
 bool
 MavlinkOrbSubscription::is_published()
 {
+	if (_published) {
+		return true;
+	}
+
 	bool updated;
 	orb_check(_fd, &updated);
 

src/modules/mavlink/mavlink_orb_subscription.h

@@ -59,7 +59,7 @@ public:
 	 * Check if the topic has been published.
 	 *
 	 * This call will return true if the topic was ever published.
-	 * @param true if the topic has been published at least once.
+	 * @return true if the topic has been published at least once.
 	 */
 	bool is_published();
 	void *get_data();