drm/mediatek: Fix an error code in mtk_hdmi_dt_parse_pdata()

We don't want to overwrite "ret", it already holds the correct error
code.  The "regmap" variable might be a valid pointer as this point.

Fixes: 8f83f268 ("drm/mediatek: Add HDMI support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>