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: NChristoph Müllner <christoph.muellner@theobroma-systems.com>
Reviewed-by: NHeiko Stuebner <heiko.stuebner@theobroma-systems.com>

lib/optee/optee.c

@@ -92,7 +92,7 @@ static int optee_copy_firmware_node(const void *old_blob, void *fdt_blob)
 
 	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);