thor: fix crash after usb initialization failure

Registration of USB download gadget might fail for various reasons, so
add a check for g_dnl_register() funtion return value. Without this fix,
thor_init() will try to access the registered gadget structures resulting
in NULL pointer dereference issue.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
diff --git a/cmd/thordown.c b/cmd/thordown.c
index 19ae672..dd0544d 100644
--- a/cmd/thordown.c
+++ b/cmd/thordown.c
@@ -37,7 +37,11 @@
 		goto exit;
 	}
 
-	g_dnl_register("usb_dnl_thor");
+	ret = g_dnl_register("usb_dnl_thor");
+	if (ret) {
+		pr_err("g_dnl_register failed %d\n", ret);
+		return ret;
+	}
 
 	ret = thor_init();
 	if (ret) {