kernel: workq: Fix type errors in delayable work handlers

A common pattern here was to take the work item as the subfield of a
containing object. But the contained field is not a k_work, it's a
k_work_delayable.

Things were working only because the work field was first, so the
pointers had the same value. Do things right and fix things to
produce correct code if/when that field ever moves within delayable.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>