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>