drm/exynos: scaler: fix static checker warning

drivers/gpu/drm/exynos/exynos_drm_scaler.c:402 scaler_task_done()
warn: signedness bug returning '(-22)'

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>