drm/rockchip: Fix uninitialized use of ret

If there are no children for lvds, ret is used uninitialized. This patch
initializes ret and returns an error if the port has no children.

Fixes: 34cc0aa2

 ("drm/rockchip: Add support for Rockchip Soc LVDS")
Cc: Mark Yao <mark.yao@rock-chips.com>
Cc: Heiko Stuebner <heiko@sntech.de>
Cc: Sandy Huang <hjc@rock-chips.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-rockchip@lists.infradead.org
Reviewed-by: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170921001408.1839-1-seanpaul@chromium.org