work_q: delayed work cancel returns incorrect status

If delayed work is already submitted or completed, then subsequent
cancel should return -EINVAL as return status.

Fixes ZEP-1373.

Change-Id: I16bbacca7e31a5a5d8e5a89e729d70302ada6223
Signed-off-by: Mahavir Jain <mjain@marvell.com>