dm: spi: Correct status register access width

The status register on ICH9 is a single byte, so use byte access when
writing to it, to avoid updating the control register also.

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