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.