usb: don't use PREPARE_DELAYED_WORK

PREPARE_[DELAYED_]WORK() are being phased out.  They have few users
and a nasty surprise in terms of reentrancy guarantee as workqueue
considers work items to be different if they don't have the same work
function.

usb_hub->init_work is multiplexed with multiple work functions;
however, the work item is never queued while in-flight, so we can
simply use INIT_DELAYED_WORK() before each queueing.

It would probably be best to route this with other related updates
through the workqueue tree.

Lightly tested.

v2: Greg and Alan confirm that the work item is never queued while
    in-flight.  Simply use INIT_DELAYED_WORK().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: linux-usb@vger.kernel.org