sf: dataflash: Fix add_dataflash return logic

This patch fixed the add_dataflash return logic,
so-that it can handle both jedec and older chips
same as Linux.

Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: York Sun <york.sun@nxp.com>
Signed-off-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>