ret maybe used uninitialised in some cases so instead
initialise it first to prevent this.
This issue was found by Smatch.
Fixes: 9671243e8d (cmd: mtd: Use the subcmd infrastructure to declare mtd sub-commands)
Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>