wlcore: Switch to using the new API kobj_to_dev()

Switch to using the new API kobj_to_dev().

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1606096062-32251-1-git-send-email-tiantao6@hisilicon.com