Unset the device user data on device off

Once we turn the device off, we may still get events from it but the pInfo
struct may not be valid anymore to process those events.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Delete the input device reference on uninit

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>