drm/panel: type promotion bug in s6e8aa0_read_mtp_id()

The ARRAY_SIZE() macro is type size_t.  If s6e8aa0_dcs_read() returns a
negative error code, then "ret < ARRAY_SIZE(id)" is false because the
negative error code is type promoted to a high positive value.

Fixes: 02051ca0 ("drm/panel: add S6E8AA0 driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180704093807.s3lqsb2v6dg2k43d@kili.mountain

drivers/gpu/drm/panel/panel-samsung-s6e8aa0.c

@@ -823,7 +823,7 @@ static void s6e8aa0_read_mtp_id(struct s6e8aa0 *ctx)
 	int ret, i;
 
 	ret = s6e8aa0_dcs_read(ctx, 0xd1, id, ARRAY_SIZE(id));
-	if (ret < ARRAY_SIZE(id) || id[0] == 0x00) {
+	if (ret < 0 || ret < ARRAY_SIZE(id) || id[0] == 0x00) {
 		dev_err(ctx->dev, "read id failed\n");
 		ctx->error = -EIO;
 		return;