drm/panel: s6e8aa0: silence array overflow warning

Smatch complains that we are reading beyond the end of the array here:

	drivers/gpu/drm/panel/panel-s6e8aa0.c:852 s6e8aa0_read_mtp_id()
	warn: buffer overflow 's6e8aa0_variants' 4 <= 4

We set the error code, so it's not harmful but it looks like a return
was intended here so lets add that and silence the warning.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>