CVE-2022-48663 gpio: mockup: fix NULL pointer dereference when removing debugfs

In the Linux kernel, the following vulnerability has been resolved:

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.