GCS_MAVLink: convert to new get_rpm() API

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -4200,10 +4200,15 @@ void GCS_MAVLINK::send_rpm() const
         return;
     }
 
+    float rpm1 = -1, rpm2 = -1;
+
+    rpm->get_rpm(0, rpm1);
+    rpm->get_rpm(1, rpm2);
+
     mavlink_msg_rpm_send(
         chan,
-        rpm->get_rpm(0),
+        rpm1,
-        rpm->get_rpm(1));
+        rpm2);
 }
 
 void GCS_MAVLINK::send_sys_status()