The desc assigned to debugfs private structure is unused so remove it.

Fixes: 9202ba23 ("gpio: mockup: implement event injecting over debugfs")
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>