drm/sun4i: Fix error code in sun4i_tcon_bind()

We accidentally passed the wrong variable to PTR_ERR().

Fixes: a0c1214e ("drm/sun4i: Add LVDS support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180115081113.wlam5wkmdynisf4r@mwanda