drm/panel/lg-sw43408: mark sw43408_backlight_ops as static

Fix sparse warning regarding symbol 'sw43408_backlight_ops' not being
declared.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202404200739.hbWZvOhR-lkp@intel.com/
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Fixes: 069a6c0e

 ("drm: panel: Add LG sw43408 panel driver")
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240528-panel-sw43408-fix-v4-2-330b42445bcc@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>