drm/rockchip: rgb: don't count non-existent devices when determining subdrivers

rockchip_drm_endpoint_is_subdriver() may also return error codes.
For example if the target-node is in the disabled state, so no
platform-device is getting created for it.

In that case current code would count that as external rgb device,
which in turn would make probing the rockchip-drm device fail.

So only count the target as rgb device if the function actually
returns 0.
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200121224828.4070067-1-heiko@sntech.de

drivers/gpu/drm/rockchip/rockchip_rgb.c

@@ -98,7 +98,8 @@ struct rockchip_rgb *rockchip_rgb_init(struct device *dev,
 		if (of_property_read_u32(endpoint, "reg", &endpoint_id))
 			endpoint_id = 0;
 
-		if (rockchip_drm_endpoint_is_subdriver(endpoint) > 0)
+		/* if subdriver (> 0) or error case (< 0), ignore entry */
+		if (rockchip_drm_endpoint_is_subdriver(endpoint) != 0)
 			continue;
 
 		child_count++;