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: 6e3c8beb ("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>