[U-Boot] rockchip: puma_rk3399: increase serialno_str size

[Philipp Tomsich]

> Increase serialno_str to 17 bytes so it can hold the 16 bytes long serial
> nummer and the terminating null byte added by snprintf.
> 
> Signed-off-by: Klaus Goger <klaus.goger at theobroma-systems.com>
> ---
> 
>  board/theobroma-systems/puma_rk3399/puma-rk3399.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 

Acked-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>