gpio: qixis-fpga: Fix error handling for devm_regmap_init_mmio()

devm_regmap_init_mmio() returns an ERR_PTR() on failure, not NULL.
The original code checked for NULL which would never trigger on error,
potentially leading to an invalid pointer dereference.
Use IS_ERR() and PTR_ERR() to properly handle the error case.

Fixes: e885002 ("gpio: add QIXIS FPGA GPIO controller")
Signed-off-by: Felix Gu <ustc.gu@gmail.com>
Link: https://patch.msgid.link/20260320-qixis-v1-1-a8efc22e8945@gmail.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>

Author: outman119

drivers/gpio/gpio-qixis-fpga.c

@@ -60,8 +60,8 @@ static int qixis_cpld_gpio_probe(struct platform_device *pdev)
 			return PTR_ERR(reg);
 
 		regmap = devm_regmap_init_mmio(&pdev->dev, reg, &regmap_config_8r_8v);
-		if (!regmap)
-			return -ENODEV;
+		if (IS_ERR(regmap))
+			return PTR_ERR(regmap);
 
 		/* In this case, the offset of our register is 0 inside the
 		 * regmap area that we just created.