gpio: qixis-fpga: Fix a NULL vs IS_ERR() bug in probe()

The devm_platform_ioremap_resource() function doesn't return NULL, it
returns error pointers. Fix the checking to match.

Fixes: e88500247dc3 ("gpio: add QIXIS FPGA GPIO controller")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Michael Walle <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Bartosz Golaszewski <[e-mail]>

drivers/gpio/gpio-qixis-fpga.c

···
 		 * create our own from the MMIO space.
 		 */
 		reg = devm_platform_ioremap_resource(pdev, 0);
-		if (!reg)
-			return -ENODEV;
+		if (IS_ERR(reg))
+			return PTR_ERR(reg);
 
 		regmap = devm_regmap_init_mmio(&pdev->dev, reg, &regmap_config_8r_8v);
 		if (!regmap)