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>

diff --cc drivers/spi/spi-rockchip.c
Simple merge