gpio: sysfs: extend the critical section for unregistering sysfs devices

Checking the gdev->mockdev pointer for NULL must be part of the critical
section.

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>