Rover: steering mode direction fix when reversing

this corrects the rotation direction when moving backwards in steering mode so that it's consistent with manual mode

APMrover2/mode_steering.cpp

@@ -3,6 +3,11 @@
 
 void ModeSteering::update()
 {
+    // convert pilot throttle input to desired speed
+    // speed in proportion to cruise speed, up to 50% throttle, then uses nudging above that.
+    float target_speed = channel_throttle->get_control_in() * 0.01f * 2.0f * g.speed_cruise;
+    target_speed = constrain_float(target_speed, -g.speed_cruise, g.speed_cruise);
+
     // in steering mode we control lateral acceleration directly. We first calculate the maximum lateral
     // acceleration at full steering lock for this speed. That is V^2/R where R is the radius of turn.
     // We get the radius of turn from half the STEER2SRV_P.
@@ -15,17 +20,17 @@ void ModeSteering::update()
     // convert pilot steering input to desired lateral acceleration
     lateral_acceleration = max_g_force * (channel_steer->get_control_in() / 4500.0f);
 
-    // run steering controller
-    calc_nav_steer();
-
-    // convert pilot throttle input to desired speed
-    // speed in proportion to cruise speed, up to 50% throttle, then uses nudging above that.
-    float target_speed = channel_throttle->get_control_in() * 0.01f * 2.0f * g.speed_cruise;
-    target_speed = constrain_float(target_speed, -g.speed_cruise, g.speed_cruise);
+    // reverse target lateral acceleration if backing up
+    if (is_negative(target_speed)) {
+        lateral_acceleration = -lateral_acceleration;
+    }
 
     // mark us as in_reverse when using a negative throttle to stop AHRS getting off
     rover.set_reverse(is_negative(target_speed));
 
+    // run steering controller
+    calc_nav_steer();
+
     // run speed to throttle output controller
     calc_throttle(target_speed);
 }