drm/panel: novatek-nt35950: fix return value check in nt35950_probe()

mipi_dsi_device_register_full() never returns NULL pointer, it
will return ERR_PTR() when it fails, so replace the check with
IS_ERR().

Fixes: 623a3531e9cf ("drm/panel: Add driver for Novatek NT35950 DSI DriverIC panels")
Signed-off-by: Yang Yingliang <[e-mail]>
Reviewed-by: Neil Armstrong <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Neil Armstrong <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/panel/panel-novatek-nt35950.c

···
 			return dev_err_probe(dev, -EPROBE_DEFER, "Cannot get secondary DSI host\n");
 
 		nt->dsi[1] = mipi_dsi_device_register_full(dsi_r_host, info);
-		if (!nt->dsi[1]) {
+		if (IS_ERR(nt->dsi[1])) {
 			dev_err(dev, "Cannot get secondary DSI node\n");
-			return -ENODEV;
+			return PTR_ERR(nt->dsi[1]);
 		}
 		num_dsis++;
 	}