Revert "[MTD] m25p80.c code cleanup"

This reverts commit 75d0ee2202b5740e94e913d8a52f91c6557c4c81.

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>

drivers/mtd/devices/m25p80.c

@@ -134,7 +134,7 @@ static inline int write_enable(struct m25p *flash)
 {
 	u8	code = OPCODE_WREN;
 
-	return spi_write(flash->spi, &code, 1);
+	return spi_write_then_read(flash->spi, &code, 1, NULL, 0);
 }