Sub: Organize descend climb_rate calc

Make it more clear to read

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

ArduSub/GCS_Mavlink.cpp

@@ -1021,7 +1021,7 @@ void GCS_MAVLINK_Sub::handleMessage(mavlink_message_t* msg)
             climb_rate_cms = (packet.thrust - 0.5f) * 2.0f * sub.wp_nav.get_speed_up();
         } else {
             // descend at up to WPNAV_SPEED_DN
-            climb_rate_cms = (0.5f - packet.thrust) * 2.0f * -fabsf(sub.wp_nav.get_speed_down());
+            climb_rate_cms = (packet.thrust - 0.5f) * 2.0f * fabsf(sub.wp_nav.get_speed_down());
         }
         sub.guided_set_angle(Quaternion(packet.q[0],packet.q[1],packet.q[2],packet.q[3]), climb_rate_cms);
         break;