drm/stm: ltdc: Cleanup rename returned value

Rename the returned value from "res" to "ret" as it is more "readable".

Signed-off-by: Philippe CORNU <philippe.cornu@st.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1500552357-29487-7-git-send-email-philippe.cornu@st.com