spi/rockchip: fix error return code in rockchip_spi_probe()

Fix to return -EINVAL from the error handling case instead of 0 when
failed to get fifo length.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/spi/spi-rockchip.c

@@ -640,6 +640,7 @@ static int rockchip_spi_probe(struct platform_device *pdev)
 	rs->fifo_len = get_fifo_len(rs);
 	if (!rs->fifo_len) {
 		dev_err(&pdev->dev, "Failed to get fifo length\n");
+		ret = -EINVAL;
 		goto err_get_fifo_len;
 	}