Using better of_property_read_u32() than generic of_get_property().

Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>