gpio: cdev: fix a NULL-pointer dereference with DEBUG enabled

We are actually passing the gc pointer to chip_dbg() so we have to
srcu_dereference() it.

Fixes: 8574b5b4 ("gpio: cdev: use correct pointer accessors with SRCU")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Closes: https://lore.kernel.org/lkml/179caa10-5f86-4707-8bb0-fe1b316326d6@samsung.com/Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>

drivers/gpio/gpiolib-cdev.c

@@ -2795,8 +2795,8 @@ int gpiolib_cdev_register(struct gpio_device *gdev, dev_t devt)
 		return ret;
 
 	guard(srcu)(&gdev->srcu);
-
-	if (!rcu_access_pointer(gdev->chip))
+	gc = srcu_dereference(gdev->chip, &gdev->srcu);
+	if (!gc)
 		return -ENODEV;
 
 	chip_dbg(gc, "added GPIO chardev (%d:%d)\n", MAJOR(devt), gdev->id);