gpiolib: use gpiochip_get_desc() in gpio_ioctl()

Unduplicate the offset check by simply calling gpiochip_get_desc() and
checking its return value.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>