video: rk3399_mipi: correct error checking

Pointers are never negative. Use macro IS_ERR() for checking.
cf. rk3288_mipi.c

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>

drivers/video/rockchip/rk3399_mipi.c

@@ -128,7 +128,7 @@ static int rk_mipi_ofdata_to_platdata(struct udevice *dev)
 	struct rk_mipi_priv *priv = dev_get_priv(dev);
 
 	priv->grf = syscon_get_first_range(ROCKCHIP_SYSCON_GRF);
-	if (priv->grf <= 0) {
+	if (!priv->grf || IS_ERR(priv->grf)) {
 		debug("%s: Get syscon grf failed (ret=%p)\n",
 		      __func__, priv->grf);
 		return  -ENXIO;