sandbox: spi: Add newline to printf() in sandbox_sf_probe

This printf() should have a newline at the end. Add it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>