optee: Replace uninitialized return variable by proper one.

As hinted by GCC 9, there is a return statement that returns
an uninitialized variable in optee_copy_firmware_node().
This patch addresses this.

Signed-off-by: Christoph Müllner <christoph.muellner@theobroma-systems.com>
Reviewed-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
diff --git a/lib/optee/optee.c b/lib/optee/optee.c
index 9b49eb8..d4355c6 100644
--- a/lib/optee/optee.c
+++ b/lib/optee/optee.c
@@ -92,7 +92,7 @@
 
 	offs = fdt_add_subnode(fdt_blob, offs, "optee");
 	if (offs < 0)
-		return ret;
+		return offs;
 
 	/* copy the compatible property */
 	prop = fdt_getprop(old_blob, old_offs, "compatible", &len);