gpio: mockup: fix resource leak in error path

If the module init function fails after creating the debugs directory,
it's never removed. Add proper cleanup calls to avoid this resource
leak.

Fixes: 9202ba23 ("gpio: mockup: implement event injecting over debugfs")
Cc: <stable@vger.kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>