tegra20: display: fix checking of return value

The calling code seems a bit in doubt about the return
value of fdtdec_lookup_phandle. Since it returns a negative
value on error (and fdt_node_offset_by_phandle as well),
check for that.

cc: Wei Ni <wni@nvidia.com>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>

arch/arm/cpu/armv7/tegra20/display.c

@@ -328,7 +328,7 @@ static int tegra_display_decode_config(const void *blob,
 	rgb = fdt_subnode_offset(blob, node, "rgb");
 
 	config->panel_node = fdtdec_lookup_phandle(blob, rgb, "nvidia,panel");
-	if (!config->panel_node < 0) {
+	if (config->panel_node < 0) {
 		debug("%s: Cannot find panel information\n", __func__);
 		return -1;
 	}