This helps when dealing with the quirky c_dfDIJoystick2 format,
which defines all the sliders as overlapping, i.e. rglSlider[2],
rglVSlider[2], rglASlider[2] and rglFSlider[2] all share the same
offset.

With the Linux backends, if the device exposes enough axes (e.g. some
HOTAS) we end up using rglSlider.

Let's make sure we don't unnecessarily override the values there.
Signed-off-by: Arkadiusz Hiler <ahiler@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>