cbfs: Rename the result variable

At present the result variable in the cbfs_priv is called 'result' as is
the local variable in a few functions. Change the latter to 'ret' which is
more common in U-Boot and avoids confusion.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>