smbios: Use char consistently for the eos member

At present a few of the structs use u8 instead of char. This is a string,
so char is better. Update them.

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