This reverts commit 75d0ee22.

Although it seems ObviouslyCorrect™, the spi_write() call uses DMA,
while spi_write_then_read() does not. Since our buffer is on the stack,
we must use the latter even though we don't actually want to read
anything back.

Pointed out by David Brownell <david-b@pacbell.net>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>