soc/amd/stoneyridge/spi: Use correct conversion specifier

Use the correct conversion specifier `z` for `size_t` to fix the error
below.

```
error: format '%lx' expects argument of type 'long unsigned int', but \
argument 4 has type 'size_t {aka unsigned int}' [-Werror=format=]
```

Found-by: gcc (Debian 7.3.0-3) 7.3.0
Change-Id: I05d3b6c9eec0ebf77cdb9e9928037e837f87ea03
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: https://review.coreboot.org/23770
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Richard Spiegel <richard.spiegel@silverbackltd.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/soc/amd/stoneyridge/spi.c

@@ -127,7 +127,7 @@ static int spi_ctrlr_xfer(const struct spi_slave *slave, const void *dout,
 	const uint8_t *bufout = dout;
 
 	if (SPI_DEBUG_DRIVER)
-		printk(BIOS_DEBUG, "%s(%lx, %lx)\n", __func__, bytesout,
+		printk(BIOS_DEBUG, "%s(%zx, %zx)\n", __func__, bytesout,
 			bytesin);
 
 	/* First byte is cmd which cannot be sent through FIFO */