gpio: mockup: add missing single_release()

When using single_open() for opening, single_release() should be
used instead of seq_release(), otherwise there is a memory leak.

Fixes: 2a9e2740

 ("gpio: mockup: rework debugfs interface")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>