spi: imx: fix error return code in spi_imx_probe()

Fix to return error code -EINVAL if no CS GPIOs available
instead of 0, as done elsewhere in this function.

Fixes: f13d4e18 ("spi: imx: Gracefully handle NULL master->cs_gpios")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Mark Brown <broonie@kernel.org>