[POWERPC] Fix windfarm platform device usage

The windfarm code uses a struct device_driver instead of
platform_driver, which can cause crashes if any of the callbacks are
called (like on module removal). This fixes it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>