usb: usb251xb: Add an empty hub' i2c-bus segment checker

It's pointless to scan the hub' i2c-bus segment if GPIOs aren't supported
by the system, since no GPIO-driven reset could be cleared by the driver
then. Moreover if CONFIG_GPIOLIB is disabled the gpio_chip structure
definition won't be available, which causes the incomplete type pointer
dereference compilation error. In order to fix this we need to create an
empty usb251x_check_gpio_chip() method returning zero, so the driver would
skip the i2c-bus segment checking and proceed with further probing in this
case.

Fixes: 6e3c8be ("usb: usb251xb: Lock i2c-bus segment the hub resides")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/misc/usb251xb.c

@@ -223,6 +223,7 @@ static const struct usb251xb_data usb2517i_data = {
 	.product_str = "USB2517i",
 };
 
+#ifdef CONFIG_GPIOLIB
 static int usb251xb_check_dev_children(struct device *dev, void *child)
 {
 	if (dev->type == &i2c_adapter_type) {
@@ -253,6 +254,12 @@ static int usb251x_check_gpio_chip(struct usb251xb *hub)
 
 	return 0;
 }
+#else
+static int usb251x_check_gpio_chip(struct usb251xb *hub)
+{
+	return 0;
+}
+#endif
 
 static void usb251xb_reset(struct usb251xb *hub, int state)
 {