gpio: cdev: replace gpiochip_get_desc() with gpio_device_get_desc()

gpio_device_get_desc() is the safer alternative to gpiochip_get_desc().
As we don't really need to dereference the chip pointer to retrieve the
descriptors in character device code, let's use it.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>