FW pos control: fix airspeed input constaining

src/modules/fw_pos_control_l1/FixedwingPositionControl.cpp

@@ -254,7 +254,7 @@ FixedwingPositionControl::manual_control_setpoint_poll()
 		 * demanding up/down with the throttle stick, and move faster/break with the pitch one.
 		 */
 		_manual_control_setpoint_altitude = -(math::constrain(_manual_control_setpoint.z, 0.0f, 1.0f) * 2.f - 1.f);
-		_manual_control_setpoint_airspeed = math::constrain(_manual_control_setpoint.x, 0.0f, 1.0f) / 2.f + 0.5f;
+		_manual_control_setpoint_airspeed = math::constrain(_manual_control_setpoint.x, -1.0f, 1.0f) / 2.f + 0.5f;
 	}
 }