spi: stm32: use NULL pointer instead of plain integer

Patch fixes sparse warning: Using plain integer as NULL pointer. Replaces
second argument of function devm_clk_get from 0 to NULL.
Signed-off-by: Cezary Gapinski <cezary.gapinski@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-stm32.c

@@ -1100,7 +1100,7 @@ static int stm32_spi_probe(struct platform_device *pdev)
 		goto err_master_put;
 	}
 
-	spi->clk = devm_clk_get(&pdev->dev, 0);
+	spi->clk = devm_clk_get(&pdev->dev, NULL);
 	if (IS_ERR(spi->clk)) {
 		ret = PTR_ERR(spi->clk);
 		dev_err(&pdev->dev, "clk get failed: %d\n", ret);