Revert "platform/x86: wmi: Destroy on cleanup rather than unregister"

This reverts commit 7b11e898.

Consider the following hardware setting.

|-PNP0C14:00
|  |-- device #1
|-PNP0C14:01
|  |-- device #2

When unloading wmi driver module, device #2 will be first unregistered.
But device_destroy() using MKDEV(0, 0) will locate PNP0C14:00 first
and unregister it. This is incorrect. Should use device_unregister() to
unregister the real parent device.

Signed-off-by: Yongxin Liu <yongxin.liu@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>