drm/mediatek: mtk_cec: Fix non-static global variable

The struct 'mtk_cec_driver' is not used outside of the
mtk_cec.c file, so make it static to silence sparse warning:
```
drivers/gpu/drm/mediatek/mtk_cec.c:243:24: sparse: warning: symbol
'mtk_cec_driver' was not declared. Should it be static?
```

Fixes: 1e914a8 ("drm/mediatek: mtk_cec: Switch to register as module_platform_driver")
Signed-off-by: Louis-Alexis Eyraud <louisalexis.eyraud@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20260429-mediatek-drm-fix-sparse-warnings-v1-3-d95c4d118b83@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

Author: laeyraud

drivers/gpu/drm/mediatek/mtk_cec.c

@@ -240,7 +240,7 @@ static const struct of_device_id mtk_cec_of_ids[] = {
 };
 MODULE_DEVICE_TABLE(of, mtk_cec_of_ids);
 
-struct platform_driver mtk_cec_driver = {
+static struct platform_driver mtk_cec_driver = {
 	.probe = mtk_cec_probe,
 	.remove = mtk_cec_remove,
 	.driver = {