gpio: mockup: fix NULL pointer dereference when removing debugfs

We now remove the device's debugfs entries when unbinding the driver.
This now causes a NULL-pointer dereference on module exit because the
platform devices are unregistered *after* the global debugfs directory
has been recursively removed. Fix it by unregistering the devices first.

Fixes: 303e6da9

 ("gpio: mockup: remove gpio debugfs when remove device")
Cc: Wei Yongjun <weiyongjun1@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>