mfd: Use to_platform_device instead of container_of

Convert mfd_remove_devices_fn() to use to_platform_device()
instead of doing container_of().

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Acked-by: Dmitry Baryshkov <dbaryshkov@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>