fs: Tell probe functions where to put their results

Rather than rely on global variables for the probe functions, pass in
the information that we need filled in. This allows us to potentially
keep the variables private to fs.c in the future, and the meaning of
the probe function is clearer.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@ti.com>