spi: sf: Print the error code on failure

Rather than just 'ERROR', display the error code, which may be useful, at
least with driver model.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>
Reviewed-by: Jagan Teki <jteki@openedev.com>