cbfs: Adjust cbfs_load_header_ptr() to use cbfs_priv

This function is strange at the moment in that it takes a header pointer
but then accesses the cbfs_s global. Currently clients have their own priv
pointer, so update the function to take that as a parameter instead.

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