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

[ Upstream commit 2d85978341e6a32e7443d9f28639da254d53f400 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>