sf: Rename sf_ops.c to spi-flash.c

Since all spi-flash core operations are moved into
sf_ops.c then it's better to renamed as spi-flash.c

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