cbfs: Change file_cbfs_find_uncached() to return an error

This function currently returns a node pointer so there is no way to know
the error code. Also it uses data in BSS which seems unnecessary since the
caller might prefer to use a local variable.

Update the function and split its body out into a separate function so we
can use it later.

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