memory: omap-gpmc: consistently use !res for NULL checks

The driver already uses 'if (!res)' pattern in the probe function so be
consistent.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200724182328.3348-11-krzk@kernel.org

drivers/memory/omap-gpmc.c

@@ -2393,7 +2393,7 @@ static int gpmc_probe(struct platform_device *pdev)
 	platform_set_drvdata(pdev, gpmc);
 
 	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
-	if (res == NULL)
+	if (!res)
 		return -ENOENT;
 
 	gpmc_base = devm_ioremap_resource(&pdev->dev, res);