spi: imx: Gracefully handle NULL master->cs_gpios

It is possible that master->cs_gpios is NULL after spi_bitbang_start(),
this happens if the master has no CS GPIOs specified in DT. Check for
this case after spi_bitbang_start() to prevent NULL pointer dereference
in the subsequent for loop, which accesses the master->cs_gpios field.
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Martin Kaiser <martin@kaiser.cx>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-imx.c

@@ -1268,6 +1268,11 @@ static int spi_imx_probe(struct platform_device *pdev)
 		goto out_clk_put;
 	}
 
+	if (!master->cs_gpios) {
+		dev_err(&pdev->dev, "No CS GPIOs available\n");
+		goto out_clk_put;
+	}
+
 	for (i = 0; i < master->num_chipselect; i++) {
 		if (!gpio_is_valid(master->cs_gpios[i]))
 			continue;