firewire: 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.

firewire core-device and sbp2 have been been multiplexing work items
with multiple work functions.  Introduce fw_device_workfn() and
sbp2_lu_workfn() which invoke fw_device->workfn and
sbp2_logical_unit->workfn respectively and always use the two
functions as the work functions and update the users to set the
->workfn fields instead of overriding work functions using
PREPARE_DELAYED_WORK().

This fixes a variety of possible regressions since a2c1c57b
"workqueue: consider work function when searching for busy work items"
due to which fw_workqueue lost its required non-reentrancy property.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Cc: linux1394-devel@lists.sourceforge.net
Cc: stable@vger.kernel.org # v3.9+
Cc: stable@vger.kernel.org # v3.8.2+
Cc: stable@vger.kernel.org # v3.4.60+
Cc: stable@vger.kernel.org # v3.2.40+