macintosh/therm_windtunnel: fix module unload.

macintosh/therm_windtunnel: fix module unload.

The of_device_unregister call in therm_windtunnel's module_exit procedure
does not fully reverse the effects of of_platform_device_create in the
module_init prodedure.  Once you unload this module, it is impossible
to load it ever again since only the first of_platform_device_create
call on the fan node succeeds.

This driver predates first git commit, and it turns out back then
of_platform_device_create worked differently than it does today.
So this is actually an old regression.

The appropriate function to undo of_platform_device_create now appears
to be of_platform_device_destroy, and switching to use this makes it
possible to unload and load the module as expected.

Signed-off-by: Nick Bowler <nbowler@draconx.ca>
Fixes: c6e126de43e7 ("of: Keep track of populated platform devices")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240711035428.16696-1-nbowler@draconx.ca